This is exactly what a young college kid in England did, but the way he did it was pure genius. He set up a 1000 x 1000 grid of pixels and sold each pixel for $1. For that $1, he put an advertisement for their business on a pixel. Any time someone clicked on that pixel, the company’s advertisement flashed on the screen. It took him four months to collect $1,000,000 from selling all 1,000,000 pixels! People all over the world would spend time playing with his grid just to see all the different companies advertised!
I rather imagine that idea is patented, but then there is always a question of how similar an idea might be for it to be allowed legally. What if you modified the idea a bit, and created a 400 x 500 pixel electronic billboards for five different cities, and allowed them to place a single unique photograph of somewhere in the city on each pixel? Then if you charged $2 per pixel, but paid the photographer $1 for each picture accepted, might that be legal? The point is to get the cities involved and their photographic community in creating their own unique billboard. Of course, if you did that for five cities, what is to say you had to stop there?