I enjoy using ImageMagick… most of it has been via PHP CLI, however here we have a short example in Bash:

#!/bin/bash
for file in `ls -d *.png` ; do
	convert -crop 1280X985+0+24 $file 
	$(basename $file .png).jpg;
done

Basically this does the following:

• Makes a listing of all .png files in the current directory
• Runs the “convert” command (part of ImageMagick) on each .png file in that listing
  • Crops the image specified by the geometry arguments
  • Saves each image as a .jpg