That's an awful puzzle and GIMP and similar tools will not help.
I've seen one in my area. It's really, really, really a bad puzzle. Requires specific hardware (not specified in the description) and requires you to open yourself to dangerous behavior on the Internet.
So here is another puzzle cache, will gimp work for this one? http://www.geocaching.com/geocache/GC20NPG_cimsagro4s-golden-mystery
Just curious
Eyeash