That's not true. They DO layer a transparent PNG on the image, but that transparent PNG has no content whatsoever. It's there just to prevent people from hitting right click and "Save Target As."
I refuse to believe quickmeme became popular because reddit liked it. That is very very anti-user.
Edit: theeeey sure do!
http://www.quickmeme.com/meme/3uwwk8/#by=ad
right click, view image, go here: http://static.quickmeme.com/media/social/qm.gif
delete that element and do it again, go here: http://i.qkme.me/3uwwk8.jpg