Cropping with GD is a good read for folks working on any sort of web application with a photo gallery. The guide describes the process for generating a cropped thumbnail from an image with PHP and GD. With this technique, one could design a gallery that uses square cropped thumbnails.

Square cropped thumbnails have two main benefits over previews that have simply been resized. An arrangment of square images looks much more neat and uniform, regardless of whether or not it contains a combination of landscape and portrait-oriented images. And since the square crop will exclude part of the image, the viewer may very well be enticed to click through to the full size version.