No way in the iPad app. If you import lists from the OSX version of Pages the image bullets may work, or get converted. I imported a list with a variety of image bullets. Some were imported, some got changed to standard number or text listings. When I continued a list (on the iPad) the image bullets were continued. However any change tot he list type loses the new images, and you are stuck with the defaults again.
So, if you have the OSX version of Pages you can probably create and import lists with image bullets, then use that document to copy and paste a few lines to new documents to start a new list. A pain, but possible.
I'm going to experiment a bit more with it. I'll let you know what works, and supply a list template if it looks to be practical.
The same may work for image bullets from Word, but I'd expect less compatibility there.