NextGEN Gallery "List" Gallery has missing thumbnails

I had exactly the same problem here. The blank spaces are caused by different sized thumbnails. I see that you have a lot of those. Also NextGEN has bug (v1.7.4) when resizing the thumbnails individually (Manage gallery > Edit thumb). The height of a generated thumbnail is sometimes off by 1 pixel. This is enough to cause blank spaces. Initially I fixed it by enforcing the size of the thumbnail in css. However a scaled thumbnail will appear blurry. Now I enforce the size of the bounding box.
(wp-content\plugins\nextgen-gallery\css\nggallery.css)

.ngg-gallery-thumbnail-box {
	width: 128px;
	height:128px;
	float: left;
}

Thanks so much, that worked very well. The reason for the different sized thumbnails is that the dimensions you set are maximums.

I’m still having problems with time-to-load and some thumbnails not loading at all ??