Pick the image that you would like tiled all over your background. Use Applications -> Image Viewer to open the file, and then save it as .xpm format.
mkdir /etc/dt/backdrops chmod 755 /etc/dt/backdrops cp yourimage.xpm /etc/dt/backdrops chmod 755 /etc/dt/backdrops/yourimage.xpm
Log out, log back in, and it should appear in the list of backdrops you can pick. The image will be an available selection for all users on your system.
Create the .xpm file as above, then
mkdir ~/.dt/backdrops cp yourimage.xpm ~/.dt/backdrops
Exit CDE, log in again, and your image will be a valid wallpaper selection for your id henceforth.