Most image conversion and editing tools usually support all the major formats but there's always a few that slip through the net.
XnView supports an amazing 400 different formats, meaning you need look no further if you've been unable to open an image in a particular format. Before you start however, note that XnView is based on the X11 platform which is basically a version of the X Window System that makes it possible to run X11-based applications in Mac OS X. Although using an application in X11 feels like going back 10 years, it does have some advantages not least that it's completely open source and therefore free to use.
You can open just about any format so that's XnView's major advantage. Beyond that though, it's hard to recommend it as a photo editing and viewing application. It takes a few seconds to process each image before it displays them and even a simple JPEG seemed to display in a very discoloured way. There are several filters you can isolate certain colors but I just couldn't get over the antiquated X11 platform to really appreciate it.
If you really can't open your image in any other application, XnView may be able to help but that's about it.
