If my Changeling transforms into an Orc for example and then casts Mirror Image, Are the Images of an Orc, or a Changeling?

I'd say of however you look at that moment.

I figured the same. I thought it might be a good strategy to transform into a humanoid enemy while close to them, then cast MIRROR IMAGE making them one of the potential targets. I know, I know, I'm so brilliant! :D 

Yeah I might allow this to work to cause confusion outside of combat, but when you're in combat you need to follow the letter of the rules on spells like this. Otherwise "creative" use leads to spells being way more powerful than they were intended to be and thus become the best solution to any problem, eliminating the need for choice which is a crucial part of roleplay.