Try typing in "Rizzo Rat" as a search on Google. And don't forget to give credit to the sites you got 'em from.
Also, if you have a scanner, you can look through some recent books for photos, and if you have screen-grab software on your computer (your best bet), you can get images directly from movies and TV shows.
Aside from that, anyone here who has images of him might be willing to share. (I have lots of Muppet pictures, but none that feature Rizzo prominently. At least not that I can think of.) Come to think of it, there's a picture of Rizzo reclining from the display at MuppetFest. You can see it on the MF photo gallery on this site. Just ask Phil for permission; I'll bet he'll let you use it.
Good luck, and let us know how it goes! We need more Muppet character fan sites.