yes, that was my first thought as well, compare filesize, dimensions and EXIF (if available). Filesize and dimensions alone should give you a factor that could be used well although there might be edge-cases where files are mislabeled as JPEG when they are in fact bmp or whatever. You should verify them by probing the file header as well

ok - i read TFA, scanned all the links blogs, their trackbacks and comments and from what i've seen there is no real info on what this is. Thinking about it for 2 minutes I had this idea that this will be best chance ever to get rid of IE6. My hope is that all the browser vendors (including MS) have conspired that maybe 3 weeks of making scary "clickjacking" news and pushing them to the main media outlets will eventually raise awareness to let go of that horrible thing that's keeping the web from really evolving. finally a good excuse to disable your content for outdated browsers that aren't patched any more because the user might accidently the whole clickjack. But in the end - if the download links don't get clickjacked that is - MS will propably release some stupid patch that prevents IE6 from clickjacking alltogether and it will be 3 more years before IE6 leaves for good....