I am working on an album with both horizontal and vertical thumbnails, Zenpage theme. On pages containing only horizontal or only vertical images, the thumbs line up and space just fine.
But on pages that have both types, what happens is I get a white space where a horizontal thumbnail would fit, followed by more thumbnails, then verticals, etc. It seems that if there is even one vertical at the end of a page of thumbnails, it somehow prevents the horizontal thumbnails from occupying what is instead seen as white space.