Hmm, I'm not entirely sure what sort of an flicker you want to get rid of, but I'd imagine it's related to the browser loading and then showing your new page with the image.
Well I can not think of any good ways, other than loading the next image into another iframe on background and then switching between the currently visible and the newly loaded iframe.
The bigger question is, why do you need to use the iframes? Are you loading something from a site that you do not own? If not, consider ajax and other such techniques.