I spent 10 minutes with Mirage and GIMP and made a dozen backgrounds. Here are 3 of them. These backgrounds work well with SLiM. I have included a scrot of the purple login.
These last three are used in wdm to replace the Debian swirl, if you choose not to use dbus, but still want a GUI login manager. You can open the file as root with any text editor. My favorite is nano, so...
with whatever color you wish the background to be.
or add a new item with a path to the image:
If you add a line, remember to comment out the swirl line. Also, drop the .jpg of your image file so that it is an .xpm file. WDM only reads .xpm files.