Using CCP and an MX-880, I *think* you can just add new png files to an icon folder (it works for me - obviously do this with CCP off).

I used to mess with the .vfd, but no longer seems worth the hassle. Not sure what the great disadvantage is of doing it this way, but now I just drop my custom icons here with no changes to the .vfd in this folder:

C:\Program Files (x86)\Universal Remote Control, Inc\Complete Control Program\MX-880\Images\Alpha\Icons\General Use

Since you're customizing, may I ask: have you found a way to replace the "waiting" images? I never could get MX-880's to use a custom wait screen.