I don't know if anyone's done this yet, but i found some images in sonic adventure's CD. Here are some of them.
BG_BIG0.jpeg
BG_CHAO1.jpeg
BG_GMA0.jpg
BG_SDJ0.jpg
BG_SXMS.jpg
Here are some more:
KB_AMY0.jpg
KB_AMY1.jpeg
KB_KNUCK1.jpeg
There's another knuckles pic, but I couldn't rip it for some reason.
KB_SONIC0.jpg
KB_SONIC1.jpeg
I promise, these are the last ones
KB_TAILS0.jpg
KB_TAILS1.jpeg