Quote the article:
Early reports show that users have been targeted as they search Google Images, one user stating that the bogus MacDefender application was automatically downloaded as he browsed images of Piranhas.
I bet it was Piranhas
... But as a bit of side discussion, for Windows users as well, this really has gotten out of control in the last 2-3 months. I often search on Google Images for fish/corals/etc, to help with ID's and stuff, and within the last few months the amount of attacks on my computer has skyrocketed. I don't even click anyway, or even get to their actual webpage (still at the webcache.google.com part with just the image).
It usually just minimizes my browser (Firefox) to a pop-up, says my computer is infected and click okay to continue. I use the task manager to kill Firefox and nothing has happened yet, but might soon.