Picadmedia's idea is simple: web publishers with lots of pictures install a piece of code, and the company inserts advertising overlays that slide up when the picture is "mouse-overed". They claim: "It [the technology] will pull relevant ads from our database and place them only on relevant images that match the same topic. It is highly accurate."
The image above is from their "How It Works" page where they show an image of a Macbook Pro with an ad in action, which makes the "highly accurate" claim look like an overstatement. If it worked as advertised, I could imagine ads promoting trips to Hawaii over photos like this on Flickr, but I wonder if the technology goes beyond the file name, the alt tag, and surrounding text in identifying content of an image (Google found it hard).
Update (Dec.8): I just remembered this old post from last year about AdBrite's BritePic that works in a similar fashion.
New Format: Ads Around Your Images (March 29, 2007)