To fix this, make a copy of the artwork (just in case), and then open the copy with a hex editor like 0xEd. Erase all the information between the beginning of file and the . just before PNG. After deleting, the file must begin with .PNG. Here are some screenshots to clarify:
- Picture #1: The beginning of the unmodified file
- Picture #2: The selection to delete
- Picture #3: The file after deleting the data

