Basically take a normal image and make it dirty by adding unnecessary censorship.
There's a few ways to go about doing this, there's the classic black boxes:
Or you can simply pixelate something:
(^ image by Lunachick71 of Cracked)
Or you can go a bit deeper and recolor something before pixelating it.
So uh... Yeah, do some stuff.
HEY LOOK HERE
Adding obvious actual porn to an image doesn't make the censorship unnecessary. The whole coloring thing I showed up there is fine, but it's just color and not an actual giant cock.