What do you think is the difference between influence and copy?

the situation with mickey mouse you described sounds more like derivative work, which is applying changes on a already-made work by others.

copying would be directly copying the image with no changes on your own. for example, like, you search for a picture of mickey mouse and directly insert it your own poster design.

influence would be of making a whole new character which is inspired by mickey mouse. there might be certain similarities between your character and mickey mouse, but one should easily see it is obviously that you didn't photoshop mickey mouse. for example, you like mickey mouse, and you made a character which is inspired by it, say, they're both mouses that could talk. when people see your art work they'd say "oh, this character reminds me of mickey mouse coz they're both mouses and look cute". but if your work is a derivative work like you said in the description, people would say "this is apparently photoshop. you just change mickey mouse's skin colour and minor details."

hope i can help. ;)