If you're willing to work on the command line anyway, you could do something like

(Where an image original-image.jpg will be converted to 400x300px and stored as resized-image.jpg)

The only advantage, of course, is that you needn't install any new software. (Although I've heard people say the gst-launch utility isn't there by default... I can't recall havin installed it, and I don't have -testing/-devel enabled on this particular device). All it does is call some functions that are part of Maemo anyway... And you could always wrap it into a script so it loops over a number of files.