Apologies in advance, I'm new to this!
I've installed the Nextgen plugin and I keep getting blank spaces in my thumbnails. I created a new gallery and reimported all the images and it seemed fine until I rearranged them and the spaces reappeared. I searched read from an old post if you change the columns to 4 it fixes it but I'm using Twenty Twelve which is responsive and it squishes the thumbs on smaller devices. You can see the problem at paulmusso.co.uk