title: Merge two pngs via command line tool in windows?
body: <p>I'm looking for a tool I can use via command line (windows 7) to merge two equally sized pngs.  I'm not having much luck with search so far as most tools I've seen are pretty heavy applications which don't seem to target simple manipulations.</p>
  
  <p>I want to overwrite the bits in the opaque base image with the non-transparent pixels in the overlay image.  I have photoshop CS4 as well, but haven't had much luck in finding the relevant command line arguments.</p>