There is no hard evidence that subliminal advertising works, but it's definitly out there. I had three or four books about this years ago. One was called, I believe, Subliminal Seduction. An earlier book, The Hidden Persuaders, by Vance Packard, came out in the 1950's.

Some types of subliminal advertising are illegal, but not all. Putting little drawings in ads is common, such as faces that look afraid, and other cartoon-like images. It's an interesting subject.