content: Haven't read the article yet but I'm sure it's not going to miss flagging a picture because someone changed a few pixels on a photo. I would assume that it would figure out percentage of similarity between pictures by looking at every pixel, comparing it to the original, and return a percentage of how many pixels are the same or close enough. It also wouldn't surprise me in this case if it would weight any flesh colored pixels as more important to avoid people just changing the background of the image.