Thumbnails in gallery are overlapping/not uniform in size

Good evening.
I am writing because I have a problem with my galleries.
I am using hte Deliciously Simple theme.
For some reason, the thumbnails, instead of all being uniform in size and square, in a grid, evenly distributed, the thumbnails are the shape of the actual image files and are overlapping/all different sizes.
https://sphotos-a.xx.fbcdn.net/hphotos-ash4/307382_4355163565591_144086484_n.jpg
that’s a screenshot i took of one of the pages with a gallery where the images are different sizes/overlapping/touching.
do you have any ideas what oculd be wrong? ive shown this to a few peeps and they havent been able to figure it out yet.
Thanks so much for your help!