Tech blog Of Zen and Computing describes how to use the free command line program ImageMagick to convert a set of images from one file format to another (like JPGs to PNGs). You can do this same sort of thing with IrfanView or Picasa, but command line-lovers will prefer ImageMagick's no-nonsense approach (and the ability to schedule the action to happen on its own). Once ImageMagick's installed and you're in the directory of JPGs you want to convert to PNGs, simply type:
What's your favourite way to batch image processing actions? Let us know in the comments.