hmm yeah you're right, the thumbnails load left to right so I'm pretty much screwed unless I have an even number of columns.
For the record, I don't think left to right is the way to go... just my .02c. Most visual people would read a grid of images downwards rather than across I think, and it really does spoil the clean layout if you're going for a grid of thumbnails with an odd number of columns.
In the end I've trimmed off an image leaving 2 cols x 4. Looks much neater, but no scrolling which is a shame.