There is option in Preferences -> Advanced:

Semicolon separated list of image names never to be used as cover thumbnails


It doesn't work out of the box. Example of what happens:

Add to that list in Preferences word "Credits".

Then for testing prepare archive where "Credits.jpg" is going to be the first file in alphabetical sorting (let's say the rest of the images is named "Superman01, 02, etc.)

So this "Credits.jpg" should be omitted when creating thumbnail for the book for the first time. It's not - browse in ComicRack to the folder of this archive and see the thumbnail. It's "Credits.jpg" instead of "Superman01.jpg".

Now strange thing - go to the Info Dialog of this archive (Ctrl+I), go to the Pages tab. It will load thumbnails. You can easily check that "Credits.jpg" has correctly marked Page Type as 'Other'. Well, now just click OK, and the thumbnail will refresh form "Credits.jpg" to the proper one.

So it kinda works but only if you enter the Info manually and let the thumbnails refresh. I hope I was understood. ;o