I had to go look for some explanations of ‘heat-mapping.’ According to this author, it’s not really a very accurate tool:

Image Error Level Analysis is a tool that creates a heat map-like image out of a JPEG that shows you the highest points of compression. Excessively Photoshopped images may contain JPEG parts that have been saved over and over again. This little tool will, in theory, detect those parts and highlight them for you. Personally, I haven’t had much luck and it seemed to think my Zuckerberg makeover was generally less altered (as best as I can interpret the results) but your mileage may vary. http://lifehacker.com/5644259/how-to-detect-a-photoshopped-image