Logo/Picture Problem

We have a site that allows users to upload photos and then we display thumbnails of them. Users can also upload logos or other pictures like gif's that aren't always the same size as photos. We wanted them all to look the same so we have been chipping them so they are all boxes (ie 100x100, 50x50, etc), but since we cut every photo in the center, we sometimes miss people's heads or miss an important part of the logo. How do other sites manage keeping the aspects of the photos yet make the site that shows thumbnails look uniform and clean?