When Voyager 2 flew by Iapetus in 1981, it found a line of white dots marching along the equator from Iapetus' bright trailing side into its dark leading side. Images shot with the white dots on Iapetus' limb seemed to suggest that the dots were, in fact, a line of peaks, protruding above the mysterious dark material. These peaks came to be referred to as the "Voyager mountains."
If you'd like to play around with the Cassini Iapetus data, I've put a page together that will make it easier for you to find images and match them up:Links to images and metadata for Cassini's Iapetus flyby
Because of the safing event that happened to Cassini when it started returning the data to Earth, the playback got a bit discombobulated. Most of the data wound up being returned, but on the JPL raw images site it's all out of order, with very few of the wide-angle context images being sorted together with the narrow-angle high-resolution shots that were taken at the same time. With a bit of sleuthing and a lot of help from Tilmann Denk's website I figured out which images go where, and also which images were missing. There are a few broken links on the page; these refer to images that Cassini took, but, for whatever reason, were not returned to Earth, or at least have not made it to the raw images website yet.