The JPG file format allows for a hidden data stream. They call data held in these hidden streams metadata. It’s meant to be used by authors for their stuff & for cameras to store any picture data. Anyway, long story short… worms have been found in JPG hidden data streams. I’m not 100% sure how they are executed… But, I suspect it will be some sort of buffer overflow attempt on the client reading the JPG file (and the metadata) & a subsequent code-injection… if it succeeds.