The only times I post unsourced stuff is when I fail to find the source and I have hopes of people in here suceeding where I failed. So far, that works 95% of the time.

Even with stuff like hentai quotes, you can still give the name of the source/artist, and that's a thousand times more helpful than just the phrase "Credit to the artist."
Again, this isn't much of a problem here as it is on, say, Tumblr or Facebook.