That Guy refers to a person who appears in the background of a photograph often without the consent or knowledge of the photogropher of subjects of the picture. That guy often takes the focus of the photo off the intended subject. That guy is often seen making a funny face or rude gesture, commonly a blowjob gesture. There is a category on college humor.com of That Guy pictures.
I took a picture of my friends at the beach but That Guy is right in the middle of it.