Please help!

I'm so frustrated right now! All I'm trying to do is create a simple lightbox. I'm also pretty new to coding, so I'm assuming there's something really simple that I'm missing. The problem is that when I click on the thumbnail, the larger image does not display correctly. Will somebody please take a look at my site and tell me what I'm doing wrong?! Thank you!

http://www.turtlecell.com/

Holly