I've been using NextGen with Highslide for quite awhile now, and although I think this problem has happened before, I can't remember how to fix it.
When I put a gallery in a post the thumbnails are getting all screwed up. Some of the rows will have 4 thumbnails, but then another row will have 1 thumbnail and the rest will be blank space. If anyone has any info on this issue it would be greatly appreciated. I am using version 1.4.3 of NextGen.
Here is an example: http://www.lincolnboyd.com/baby-bash/2009/12/31/