I bought a book from Kobo that had not just the cover, but also the first few pages of text displayed in full-screen mode.
I had a look inside the epub and found that the files hadn't been split properly, and the text that was displayed in full-screen mode was in the same file as the cover. I think the kepub reader is just taking whichever file contains the cover and displaying the whole file in full-screen mode.
So perhaps you can use this to display your images full-screen by putting them in the same file as the cover image. (This was the book:
http://store.kobobooks.com/en-nz/ebook/aristoi-1)