PLEASE!!! tell me we are in sweeps again.
The author waits until the very end to state "an infected JPG file cannot infect another computer on its own"
Go McAfee's AVERT page and you learn that its a standard virus.
With a standard infection vector of a executable that looks for trigger data in a picture file. No different then one that looks for specific files, dates, URLs, etc. It is just looking in a data file for the trigger then just the presence of a specific type of file.
The only thing different is that the trigger data contains the code for what action to take.