I've got a few more ideas about what to make, but I still have a few questions. Is it a problem if I made them in a different size (512x512 instead of 256x256) and put them into the same package as a 256x256 file?
You need to resize the thumbs to be 64x64. The reason for having thumbnails is that loading lots of large images takes a long time. Having thumbnails that are the same size as the regular image defeats this purpose.