Images of children involved in explicit sexual behavior is, of course, child pornography, but sexual activity does not have to be pictured for the images to be considered pornographic. In some jurisdictions, images depicting children nude or in erotic poses can be considered child pornography.
The U.S. PROTECT Act of 2003, made virtual child pornography illegal, that is pornography in which no child actually participated, but is created so that it is indistinguishable from actual child sexual abuse.
Simulated child pornography has also been made illegal in some jurisdiction. It is pornography in which photographs of real children are altered to make it appear they are involved in sexual activity or photographs of older models made to look like children.