I'm not sure if the optimal dimensions are different, but this collection works well on a G1.

And here is another nice set.

zedge they are available in the app store

I've found that simply doing a Google Image search and setting the options (click the "Show Options" link for the sidebar to pop out) to the correct dimensions for your wallpaper can result in pretty awesome background finds.

That way, you can search for whatever you're interested in and get a great wallpaper according to your tastes.

Of course, you could go further and find an image you like, and use a host of free online image editors (like Picnik) to take any interesting image you find and cut it down to size.

Shameless plug: The Guardian publishes lots of really nice photo galleries. The Guardian Anywhere app can use any of those photos to set as wallpaper.

As an example, I quite like the wildlife galleries.

Nasa wallpapers in the marketplace has a good variety of some beautiful pics

Free app in Market simply called "Backgrounds". It is great. Zedge was cool too. But for me, Backgrounds has been the best by far for my Droid.

Side Note: I loved the wallpaper pict I used on my Treo. It was a fancy logo of my Fave NFL Team. It stretched funny across the Droids 3 screens no matter how I edited it, but I was determined to use it. My answer - a widget. Long press on home screen > click widget > scroll down to "Picture Frame" (Standard on all Droids) > click it & it will take you to your gallery > just choose the picture you want >slide the frame how you want it etc... then once done you will have a 2x2 picture framed on your home screen. It is a cool compromise ;o)