This has been SOLVED (see 2nd comment below)
 
When the slideshow is playing, the thumbnail tray highlights the thumbnail of the current picture being shown; this works fine for the first row of thumbnails, but when the slideshow gets beyond the end of the thumbails currently showing in the thumbnail tray, the thumbnail tray does not automatically advance to correspond to the next set of thumbnails that the slideshow has reached. This appears to be an oversight, since users shouldn't be expected to have to manually page through the thumbnail tray in order to keep up with the slideshow's progress through a set.
 
And if anyone has already accounted for this in their own code, I would absolutely LOVE to see your code so I can fix mine too!
 
Thanks.