Creating a gallery

Hi all,
I’m having trouble creating an image gallery that shows up like it’s supposed to. As you can see in the link (http://suzannebirch.com/gallery/), there are big white gaps showing in between the rows. I’m using the theme Portfolio by ThemeHorse, but the problem occurs in every theme. On the theme preview, galleries show up like very nice Pinterest-style layouts or square grids. But Im not able to make my own gallery look like this.

Any thoughts?
Thanks in advance 🙂