I'd like to see how it works on the third image from the top, with the gas station sign, etc. It is fairly straightforward to detect a billboard in a random nature scene; look for straight lines and intersections of them. Most (all?) billboards are also horizontal. It is quite another thing to detect billboards in an urban setting.
I've added the output at the end of the blog post for that image... check it out!
This algorithm works pretty well for billboards in urban settings as well -- since the photos come with the units outlined.