I thought about this then I remembered I have a soul and like to better the world.
While an interesting idea I know I wouldn't use it if it was an ad. Despite the problem of my sample images trying to sell to me it would be annoying when trying to mockup a design (with images that randomly have undesired colors, patterns, etc.)
It's a solvable problem, and with the right targeting could actually be useful. How many people would discover Basalmiq for the first time via the placeholder image on their WIP site?