I hope to have said otherwise, but in my opinion the best image/photo management software for Linux is Google's Picasa. If you haven't known yet that Picasa is available for Linux, well so did I. It was surprising for me, but I was really glad when I found out about this.
F-Spot and DigiKam is good, and some others may provide better image manipulation features (in that regard, you can even use ImageMagick). But let's face it, Picasa is pretty in many ways. It's UI is superb. It's smooth. It's fast. It's slick looking. It's useful. It has many common photo touches that most people would ever need. Some people may complain it's too newbie- or beginner- oriented but I'm not the one who does, especially since it's available for Linux.