Not sure what or where you mean it was "listed under Sansa", but yes, if you follow the instructions for setting the background using a bitmap of the correct size it should work. Â Oh, and for the screen size, I don't know it by heart but it's available all over the place, poke around in the wiki.
p.s. Once you have the image ready, you can set it in Rockbox by doing a long-select while the bmp is selected and choosing "Set as backdrop".