CDE comes with many built in backdrops. These are low color and relatively plain. You can set your own backdrop easily.
You can add your own pictures to the list that is in Style Manager. They can be any image in XPM or XBM format, named as .pm or .bm,.
Place your backdrop in
$HOME/.dt/backdrops (for one user) or
/usr/dt/share/backdrops (for all users) and when you next open the Style Manager Backdrops you will see it added to the list of backdrops.
For full-screen backdrops the images should match the size of the desktop, else CDE will tile the image if it is smaller.
First, in the Style Manager Backdrops, select NoBackdrop and click apply. The backdrop will now turn black.
Next using the backdrop setter of your choice (ImageMagick, feh, xli, etc) set your backdrop. Make sure to add this to your autostarted applications. Logout and back in.