New version. Now pictures scale automagically to the screen size. This means it is usable to display pictures taken with the built-in camera, which is a higher resolution than the N900's screen. I also added a few lines to keep the screen from blanking while the slideshow is running (thanks to MartinK!)

Also I have discovered that this runs on regular (linux) computers that have the necessary libs installed, with no modification. I still have to finish the configuration bit though; for now, just run it from the command line.