Thumbnail Grid Disrupted

I'm sorry if there is another post that covers this issue, but I searched using every description I could think of and couldn't find anything. The issue is that the thumbnail images on the Gallery Thumbnails Text Bottom exhibit layout "pop out" of their position on the grid/gallery. Here's a screen shot showing the best arrangement I've been able to come up with, but note the three images "popped out" on the bottom row:

The small-looking image in the top row is a scan done horizontally instead of vertically, and I've tried excluding it from the grid, but it doesn't seem to make any difference whether it's there or not. Thanks for any clues.