So in my last session, a spellcaster uses Silent Image to create a wall that covered an alcove he was in.  His objective was to use his Wand of Magic Missles to cast it through the Silent Image when he sees the guards walk by or within range of the wand. 

A friendly discussion came up where one opinion, mine, said since it is a "The image is purely visual; it isn't accompanied by sound, smell, or other sensory effects." that is looks like a wall from both sides, and not just one-sided, so effectively what the spellcaster sees is a wall in front of him and no way to determine when the guards are within striking distance of the wand.  and since "You can use your action to cause the image to move to any spot within range." I felt that this is a 3-d image and not something flat since the movement gives more depth to its movement.

The counter was from the rule that states "If a creature discerns the illusion for what it is, the creature can see through the image."  that since the spellcaster 'cast' the spell he knows it is an illusion so he should be able to see through the wall and attack when necessary.

So my question is when it comes to illusions what does the spellcaster see?   

As a DM, if I'm ruling on this, I'm going to rule that the caster, as the originator of the illusion, knows it is one, and can see through it. I might impose a Disadvantage on perception rolls to look through a completely obstructing image like the wall. For instance, if the image was of a person standing there, it doesn't obstruct the entire hallway, so you could just see through it no problem. But an image of a wall covering the doorway would obstruct the entire surface area, and even though you can see through it, the text does not say that the illusion is invisible. It would be like seeing through a curtain... you could see what is beyond it in a general sense, but there would be some penalty to discerning details. For instance you could probably tell that a tall, human-sized figure is coming toward you but would be at a disadvantage to tell if it was a human or an elf through the illusion.

I would rule that the caster could see through it. And since the spell does not mention the seen through spell obscures anything behind it any more, I would also rule that the caster (and anyone who recognized the illusion) can see through it clearly.

I would make the guards very aware of the spell that shot through the wall though and either give advantage or just immediately notice the illusion.

I require all creatures to "discern the illusion for what it is" using either method mentioned earlier in that paragraph; I don't think it's a coincidence those sentences are placed together in sequence. The alternative interpretation is unsatisfying to me and too prone to gameplay cheese and arguments. This cuts both ways; enemies that recognize the spell as it's cast or figure the caster couldn't have possibly cast Wall of Stone because they're an Arcane Trickster or whatever don't automatically disbelieve the illusion. The caster's allies don't get a free pass just because they let them in on the spell either.

The way I see it, the spell's going to fool your senses/subconscious no matter how strongly you believe it's fake until you witness evidence to the contrary. But there's definitely room to interpret the spell both ways.

I would allow the caster to see through the illusion. They made it, so they know it's an illusion. 

I wouldn't add any penalties to looking through an illusion. The spell doesn't say anything about that. You either believe it, or not. In my games, if you notice the fake magic, you ignore it or it passes from your mind.

So if someone is attacking with a range weapon and you are hidden behind an illusion crate that looks to provide 1/2 cover it’s not like it actually provides the cover.  How would I roll the attack in case like that and again assume person being attacked does not get cover bonus. 

Would they not get the bonus?

If I assume your lower half of your body is behind, say, a barrel, then I am going to aim for your head/shoulders. That is a smaller target. It is harder to hit. I am more likely to miss high or wide.

After I wrote the comment I came across this. 

https://www.google.com/amp/s/www.sageadvice.eu/2019/01/13/hiding-behind-an-illusion-is-not-the-same-thing-as-having-cover/amp/

Chris Perkin states the following  though I’m having a hard time understanding   

Hiding behind an illusion is not the same thing as having cover. A real wall provides cover. An illusory wall does not, though it can block line of sight (which is not the same as granting cover)

If a creature is heavily obscured and has cover, it gains the benefits of the cover and the benefits of being invisible, which is why cover doesn’t use the advantage/disadvantage mechanic but invisibility does.

Yeah, illusory cover is no cover. Unless it is total cover, then they at least have unseen target disadvantage, but they are also shooting at a wall, and why would they do that unless they knew it was an illusion?

Yeah, I changed up the scenario to say if there were behind an illusion crate to create that cover question.

It comes down to what they mean by "see through it".

It could, of course, mean that it literally becomes transparent to those who know it is an illusion. However, there is a common use of that phrase in terms of "seeing through" a deception. If taken that way, you'd be able to tell that it was an illusion, you've "seen through" it and know it is an illusion, but the illusion is still present. That's the way I first read it. It would be like seeing a disguised person, and realising that they were wearing a disguise. You know they are in disguise, but you don't instantly see all the features behind it to know the true appearance of the disguised person.

That said, I'd allow the caster to see through it. However, if something came shooting out of a solid wall, I'd expect an enemy to be able to tell something fishy was going on, so at the very least gain a bonus on trying to see that it was an illusion. As soon as one knew, he could inform all the others so the illusion would become transparent to all the enemies.

It comes down to what they mean by "see through it".

It could, of course, mean that it literally becomes transparent to those who know it is an illusion. However, there is a common use of that phrase in terms of "seeing through" a deception. If taken that way, you'd be able to tell that it was an illusion, you've "seen through" it and know it is an illusion, but the illusion is still present. 

That is how I interpret it as well. Which is why I said I might impose some sort of disadvantage toward being able to tell details on the other side of a large, grid-square-spanning illusion even though you know it is an illusion.

Quote from BioWizard >>

It comes down to what they mean by "see through it".

It could, of course, mean that it literally becomes transparent to those who know it is an illusion. However, there is a common use of that phrase in terms of "seeing through" a deception. If taken that way, you'd be able to tell that it was an illusion, you've "seen through" it and know it is an illusion, but the illusion is still present. 

That is how I interpret it as well. Which is why I said I might impose some sort of disadvantage toward being able to tell details on the other side of a large, grid-square-spanning illusion even though you know it is an illusion.

 Almost like looking through a stain-glass window.

I think of it more like looking through a Star Wars hologram... it's faded and it's flickering. But if you fail to detect it the hologram would look completely solid.

I was looking at this to see if an NPC Blastlock could hide behind an illusion and so gain advantage on EB-ing the party.  For me, the key line in the spell description is "Physical interaction with the image reveals it to be an Illusion, because things can pass through it.", ie, if something passes through it, it is automatically revealed as an illusion. The Investigation check is only required in the absence of physical interaction.

I interpreted that to mean that the Blastlock could see through the illusion by putting his hand through it, making it clear to him that it was an illusion, and so making it faint to him.   If the party did not notice him doing so, it would remain convincing to them.  However, I considered casting a visible spell like EB through it 'physical interaction' that the party could see, revealing it to be an illusion to them as well.  So, it would work, but just once.  Give the Blastlock the Devil's Sight invocation and put him in the AoE of a Darkness spell, and well, now you're talking...