I kept coming across these ad overlays on images today, and they led me back to Image Space Media, the company behind the format. AdLab first wrote about them last year when they were still known as PicAdMedia. AdBrite has also tried a similar approach in the past, and the service is still active.
While potentially interesting, it doesn't seem like an easy format to get right. Serving messages relevant to the image is key, for which you need rich metadata or some kind of image recognition algorithm, otherwise you are going to end up with an image-ad combination like the one below, on the company's own site (click image to zoom).
On the other hand, perhaps an ad for an ad blocker served as a layer over "We Love Advertising" image is a perfect match.