I've made t-shirts for my boys the last two years we've been down. I have used scans from some Disney books I own, images from commercial software programs, and some images from the internet. All of the images I got off of the internet were not Disney: they were some movie still shots for some other theme parks we were going to visit. I decided the Disney clipart on the internet does not enlarge well for my needs.
Anytime a CM noticed one of my boy's t-shirts, they always made a favorable comment. No one at WDW seemed to mind that I hand made some Disney themed t-shirts so that my kids could show off their love for Disney.
One software that has quite a bit of images is Disney Print Creations, put out by Sierra. I think it cost around $40. By the way, the box may say 6,000+ images, but they take color images, then do them in black & white, then do them in shade, then do them in outline, then do them in silhouette, then do them in pastels....you get the idea. But it's still a pretty great collection of Disney graphics.
Whatever you decide to use, have fun!
