Quote from igneousguy >>

"players are using the spell (as walls, doors, images of themselves)" ?

You cannot create a minor image of a creature.  Only a stationary immovable object.

 A stationary, immovable object like... a statue of myself or another creature?

A realistic manikin is what I usually do. My illusionists usually take the Keen Mind feat. They visit all sorts of shops and memorize the appearances of many things like mannikins. They tend to practice these illusions regularly to keep from forgetting the images after a month.

I will also say remember whatever they know in the real world, they are dictated by there in game stats, I used to play with a physics professor who was amazing because he never thought above the int of his character. If they are not high int talk to them and explain they just don’t know that stuff in game. 

Quote from Mergon >>

A realistic manikin is what I usually do. My illusionists usually take the Keen Mind feat. They visit all sorts of shops and memorize the appearances of many things like mannikins. They tend to practice these illusions regularly to keep from forgetting the images after a month.

 most humans and humanoids will be slightly taller than what is allowed by the spell, yes you can say that the imaginary cube is tilted 45 degrees and thus gain more room but that is kind of working against the spirit of the spell

By you reply the chair couldn't be setting on the floor.

In my experience minor illusion isn't that much of a problem for  a few reasons

1. a silent non moving image of a humanoid is unlikely to draw much attention in combat
2. any interaction reveals the illusion  to be false. It is unclear who it is revealed to but a common interpretation is everyone aka the illusion is functionally dispelled if attacked or attacked through
3. Casting minor illusion requires an action the same as discerning it meaning that it is typically  neutral in terms of relative action economy. 
4. hiding still requires a separate action 

it's one of those things that sounds like it should work better than it actually does. In combat most enemies will functionally ignore a minor illusion of anything other than an inanimate object and any inanimate object minor illusion can at best be used for a single attack with cover.  If used to hide then it takes an action to see through the same as searching the area normally. 

I generally run illusions as follows

• illusions/ images end if attacked or if an attack would pass through the image 
• phantasms can last multiple hits but require a save
• minor illusion reliably depicts inanimate objects only. It lacks the ability to convincingly act as anything moving 
• Silent image can do inanimate objects but not phenomenon like fire or water. It can also trick people in combat but won't hold up as a living thing in conversation
• Major image can reliably trick people  in conversation  and pass as any phenomenon.
• I also let image spells work like a veil of invisibility. Things inside the area of an invisibility spell can be made invisible to those outside of it but this effect ends when some one else enters the area (as they appear to vanish).  Silent image  and minor illusion cannot hide sounds smells ect when doing this but major image can. If you have the enhanced minor illusion ability you can use this to hide sight and sound but it's still limited to a 5ft cube. The silencing effect also just an illusion and doesn't prevent spell casting or thunder damage. 

It's important that minor illusion can do less than silent image and that silent image can do less than major image. In particular silent image and minor illusion need to be significantly limited because infinite uses of both are easily obtained. This means that allot of the shenanigans people like to do most with illusions are limited to major illusion but given how impactful  something like convincing a whole town  with a speech given by an illusion of their king can be, I think it's fair that it require a third level slot.