Here's my favorite deceptive optical illusion billboard from a collection found by the site Mighty Optical Illusions (which was found via Artatm.) Not sure how well this works as advertising, since the logo for Berger "Natural Finish Colours" is so small, but this billboard would get lots of attention.
What's actually happening is the supposedly rectangular billboard has been altered by cutting out pieces in the upper right corner and making thin pieces of white billboard stick up.
It's very difficult not to see the larger roller strokes as dominant, especially because the painter with his shadow sticks out. I can't tell if he's been painted or is a dummy figure. I also don't know how they get his paint roller to appear correctly. Maybe it's transparent?
This optical illusion plays with our assumptions - we think:
1) That must be a guy hanging there because he looks real and has a shadow (he's not real.)
2) Billboards are rectangular in shape (this one is not.)
3) Paint when applied to a surface covers something (this is not paint, and it doesn't cover but reveals something - in this case the sky.)
4) The wider "brushstrokes" catch our eye and seem closer to us (yet they are the ground - the background - and not the figure or foreground.)
5) Much of the billboard - the rectangular billboard surface, the guy hanging there, the painting with a roller - is possible and not fantastic, so seeing this we might at first assume that there's someone up there painting a billboard. The only odd thing is that he's painting the billboard the exact same color as the sky... which is impossible.
Here's the same photo manipulated to show a darker-colored sky:
Billboards from the great site Mighty Optical Illusions>>