Well, it could be done in whatever image editing program you use. Open the file in Paint or Photoshop or GIMP or whatever, then go to file, and "Save as". You should get a menu where you name the file, below that a drop down box. Click the drop down box and find "PNG" Then give the file a new name and click "ok"! Of course, if you have a Mac I have idea since I'm a PC kinda gal.