Pics provide instant gratification. They are also usually low on intellectual info. And thus pic-only submissions are mostly flagged/removed on HN from my experience.
I don't know of any studies but I, for one, would be much more inclined to click on a [pic] link compared to a text one, given the topics are of nearly equal interest. The reason is, you can judge the relevance/interesting-ness of a photo at a glance, comes in handy when you want to review a lot of entries.
The photo is on imgur so it should be dead soon on imgur as well if this is true.
http://rapidshare.com/files/397708304/11DLu.png.html Just in case :)