CDE comes with a generic logo for the Dtlogin screen. You can customize this to your own personal taste, or if you are the creator of a distribution, your distribution's logo.
The default logo is in /usr/dt/appconfig/icons/C/Dtlogo.pm
Make a backup of this file before you start.
Next, using the graphics program of your choice, create a new logo. It must be 239x239 pixels.
When you are finished, save your log as Dtlogo.pm. It must be in .pm format. Dtlogin doesn't accept any other format at this time.
Move this new Dtlogo.pm to /usr/dt/appconfig/icons/C/ then logout. You'll see your new logo!