You are not replacing over existing images with fanmade renders or increased sizes. There is nothing wrong with them. They are big enough, and you cannot go any bigger than 3000 pixels. You can do these things on Wookieepedia, but things are different here.
Due to some growing concerns about the freedom there is in the imagery department, new restrictions and limits were put into place to insure order. Make do with what we have.
I'll help out if you have some specific image requests. But much of my time is taken up at the Agents of SHIELD wiki, where I'm now and admin. But I'll try and get over her to work on those articles here now and again.
Apologies. Didn't notice this. I'm used to seeing a yellow banner across the top of the screen when I get a new message posted to my talk page (like Wookieepedia) from other Wiki's than the one I'm currently working on. I don't actually own any of the Toy Story movies on DVD or blu-ray to be able to get my own screenshots, so it'll have to be from internet scouring. Let me see what I can find.