I also would LOVE to have a solution for this. When there are too many thumbnails to display in a row, there is no way to scroll the thumbnails to the right or left to jump forward. I tried loading 100 images, which worked beautifully and quickly I might add (NICELY DONE DEVELOPING THIS TOOL AND API!!!), but to get to image 50 I had to select the far right image, wait for it to load. Then the thumbnail bar would slide all the way to the left, as I would expect, and then I clicked on the far right thumbnail again. Waited for it to load and then the thumbnails all slid to the left again. And so on, until I finally got to the 5th screen of 10 thumbnail images, where I finally found number 50.
I think Mark's solution is perfect. An arrow that appears when your mouse reaches the far right or left of the row of thumbnails would be perfect. Just like the arrow on the slides themselves.