IIRC, you can resize the image proportionally in Photoshop by entering a percentage. The program uses pixel sampling to reduce the image size while retaining a relatively high degree of detail, so that the file itself shrinks with no appreciable degradation in image quality. Can't remember what menu you find that under.

Or if you're like me or Dub you can open the image in Microsoft Paint and choose "Stretch/Skew" from the Image menu, enter 50% (or whatever you need) and resave the image.