I had never seen the show, but I gathered a lot of information from fan sites. Dexter works as a forensic bloodstain pattern analyst who moonlights as a "good" serial killer. As of Season 4, he is the single father of a baby boy named Harrison (whose mother was murdered). Having been trained to commit perfect murders by his adoptive father, Dexter is becoming more concerned that his son Harrison might be following in his footsteps. In season 5, there is a humorous and telling scene where baby Harrison pops the head off of a Ken doll just after a therapist claims that a 10-month-old is incapable of mentally processing his mother's murder.
In this poster, I tried to play on the idea of Dexter being a father/serial killer by using a baby mobile complete with common murder weapons employed by Dexter. The decapitated doll head in the center alludes to the aforementioned scene as well as several of Dexter's beheaded victims. The blood represents, well... blood.