Jetpack Square Tiles not uniform

Hi

I’m using Jetpack square tiles for my galleries and I’ve noticed that on some the pictures are all uniform but on others they are not. I thought this might be due to irregular pictures sizes and resolution, so I messed about with those to no avail.

Couple of link for you to see what I mean:
http://flippingthepyramid2.rootslion.com/music/reggae/johnny-osbourne/
http://flippingthepyramid2.rootslion.com/music/hip-hop/akala/

Any help much appreciated.

Thanks