[Plugin: nextgen-gallery] Gallery display problem

I’m using NextGen-Gallery 1.2.1, and I’m getting a layout problem with my galleries. The thumbnail images won’t line up correctly in grids, leaving seemingly random gaps. Seems to happen in both IE and Firefox. I’ve tried redoing all my thumbnails in case I’ve inadvertently sized them wrongly, to no avail.

It’s probably easier to show that describe. Here’s an example.

Can anyone help?