They are not available outside of Dell's factory image.  There are some really cool ones out there though ... I think you'll be glad not to have to stare at Dell all the time :)

Actually, the gray login background image (with the curvy lines) is included (or it was with mine, anyhow). You just have to re-enable the OEM background and copy the backdrop image that you want to see -- there's a choice of several different colors.  Do a Web search for windows 7 oem login background. There are lots of pages describing how to do it: a registry edit is needed. You'll have to do that anyhow if you want to substitute your own login background for the garish green one that MS provides by default.

The user theme which includes the runtime background image is what's missing. You'd think Dell would want to advertise their systems by making it publicly available. :(

s.