This is completely controlled by the CSS.
In the absence of a specific definition, the div will generally expand to fit the photo. The easiest fix, I think, is to use photos cropped to the correct size.
However if that's not convenient, you can define the photo height or width in the css and the second dimension will adjust.
Another possibility is to control the overflow and the zindex to hide anything bigger than allowed div size. That's a more advanced technique and it depends on your skill level. THere are quite a few techniques, and I'm not sure which ones would be best at this point.
Here's one reference I found to a similar question, it might give you some ideas.
stackoverflow.com/questions/5485341/css-...-on-image-size-above I'll also ask Tessa if she has some insight. She's better at CSS than I am.
Cheers,
Ed