The Mystery Picture is not an entire image, its a 10 pixel by 10 pixel frame of an image section. To help identify it, look closely at colours, outlines, textures and individual pixels. Even a tiny colour change can help narrow down the source image! Remember, the snippet can come from anywhere on Neopets!
Pro tip: Give that weekly item prize a closer look, sometimes it drops a subtle clue to help you crack the case! Happy hunting!