Problem with images on my Homepage (autofocus theme)

Hi everybody !

It’s been a while that I’m searching on the forum but I don’t find how to solve my problem. I’m using the Autofocus wordpress theme.
I’m a newbie with CSS and I’m not sure how to change the codes…

My problem is:
On the homepage a few pictures are displayed (coming from my posts’ galleries) but those pictures appears like they’re zoommed so they don’t fill the original layout.
I’d like to see kinda preview of my picture (even if it’s cropped) and not only a small part of it (as my pictures are in a large size I can only see a piece of background on the preview). I guess I just need them smaller. How can I change those picture previews?

Sorry I don’t use any technical vocab… I just don’t know how to call all those things 🙂

Many thanks in advance!