Took a look at it with a hex editor and I saw that it was nothing more than what Freeplay has already explained how to alter, and a simple png file made in photoshop.
Notepad will NOT work. Open the prs with your hex editor, and remove everything down until %PNG (so %PNG is now the beginning). Then go to the end of the file, and remove the white space. Save the file as any png.
Now you can use your brains to reverse the method and start making your custom icons!