If you have root:

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.

If you don't have root:

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.