This is not the best stegano I've seen, at first I expected too much, then I've read the hint of already having the right tool on my computer, so I thought it's either some basic image manipulation or I should open the image with a text editor.
damn, first i open the file with a hex-editor - saw the link at the end, but there was nothing what help...
then i try to find the solution in the squares...nothing
next day - solved by opening with hex-edit again
You must be shitting me. I used OS X preview to filter colors in the picture and look for patterns. What I didn't know was that this automatically saved the changes to the picture. When I later was on the right track the answer was gone from the file. I later spent a whole day afterwards printing out the pixel data as ascii characters in different configurations before I noticed that the picture I was working with was not the same as the original. 1 minute later I solved it. At least I now know how to python gif files.