Sorry, I should have taken my own advise and looked at the programming manual.

To add graphics files:

Go to C:\Program Files\Universal Remote Control, Inc\Complete Control Program\MX-880\Images\Alpha\Icons and add your graphics.

Then, when selecting "Devices" under the "Program" tab, the added images should be available in the display of images to the right. You then have to create a new device with the image. These buttons can be then copy/pasted to other pages and devices. You can then delete the created device.

It's clunky, but it works. The MX-980 is much easier, as you can use custom graphics for any button just by importing them for each button.