It's possible. I would assume you could edit the cache that Dolphin has (or hack Dolphin itself to display an image of your choice). Another way would be to edit the banner data on the ISO itself. Here's a good start for info: http://hitmen.c02.at/files/yagcd/yagcd/chap14.html
But of course it's would be best to look through Dolphin's source code for a more complete reference.
But of course it's would be best to look through Dolphin's source code for a more complete reference.