This is a pretty interesting picture, and it spawned all types of speculation and fanfiction about Dexter's Laboratory being a prelude to The Big Bang Theory. I was going to mention the questionability of Dexter being Leonard and Dee Dee being Penny, but Wickfield, the Deviant Art creator of the Dexter's Laboratory image, had this to say about the incest implications:
AND NO IT'S NOT SUPPOSED TO BE INCEST, PUH-LEASE. I just put Dexter as Leonard because he's the main character (and shortest), and Dee Dee is Penny because she's the sole blond girl. It's not supposed to be an exact copy, but everyone on Tumblr is freaking out about that and THAT'S THE JOKE. THAT IS WHY DEXTER IS 0.0. Read the description, that's why it's there.
Dexter: "Dee Dee, this is not the Big Bang theory I thought you were talking about!"
Dee Dee: "Oh well - a little dress-up's always fun!"
Mandark: "Yeah right, especially when your own brother gets to date you before I do...."
The picture to the right is a promo picture for the complete second season of The Big Bang Theory. The Dexter's Laboratory comparison was drawn by Wickfield.