You mean the image that you see when you choose the campaign???
When you are building it, it goes in the RESOURCE\IMAGES directory and is referred to in the .mcf as it would for an image placed in the briefing of a single mission... It then gets embedded in the .MCF just as the images get embedded in a .cmf file.
Err...... At least I THINK I remember it this way. I will double check tonight and post a correction if necessary.
I'm pretty sure that only TEXT files get compresse with MAKECAT into the Ctexte.cmp file.