Flutterby™! : Hacking JPEG

Oooh! Cool idea! JPEG canaries: exposing on-the-fly recompression.

Our algorithm works by adding a high-frequency pattern to the image with an amplitude carefully selected to cause maximum quantization error on recompression at a chosen target JPEG quality factor. The amplitude is modulated with a covert warning message, so that foreground message blocks experience maximum quantization error in the opposite direction to background message blocks. While the message is invisible in the marked original image, it becomes visible due to clipping in a recompressed copy.

What's also cool is how their sample images look as you scale 'em on the iPad.

Hat tip to MarkV.