If it exists, there is porn of it. No exeptions.
From 7chan. Often users will take the miccy and send things gross, such as 100+ adult pornography, or some sicko would say "CHILDREN UNDER 10! RULE 34, GOOO" often with an awful lot of spam to show how stupid they actually are.
OLD GRANNIES BEING SHAGGED, RULE 34!
Guy: I watched power rangers last night, the pinks one's hot
Guy2: RULE 34 ON TEH BLUE ONEZ!
Guy3: *Posts a picture saying "GTFO"*
Guy: Nice one, Guy3!