Leroy Stephenson, 24, faces felony charges of aggravated criminal sexual assault and child pornography, the department said.
Investigators said the case was brought to them after the girl's mother reported that a photo of her daughter in a sexual act with an adult male had been posted to Facebook.
Stephenson told detectives that he had been involved in an ongoing sexual relationship with the 15-year-old since February 2010.
The victim confirmed the relationship, telling detectives that she gave in to Stephenson after he had pursued her consistently over the course of two years
Police say Stephenson admitted to taking the photo, and investigators believe he used his cell phone to send it to another individual. The photo eventually reached the phones of 10 other people, and from that point the photo was posted on numerous Facebook pages.
The family of the victim has asked Facebook to take down the multiple photo postings.
Detectives notified the Department of Children and Family Services, which also is investigating.
Stephenson has been ordered held on $75,000 bond during a Friday court appearance in Markham. His next court date is Feb. 3.