Banning virtual child porn based upon what a pedophile might do with it is like banning guns because of what a bank robber might do with a gun.
A careful reading of the USSC decision (that Timesink was kind enough to post for us) makes it clear that as long as the image is not "actual," ie. photographic, it is legal to publicly display it. The billboard image I hypothesized, while closely approximating a photographic image of the real President, would likewise not be an "actual" image. By the SC's own interpretation, the sponsor of this display could not be prosecuted for the crime of threatening the President. (As a sidebar, consider what happened to Craig Kilborne when he displayed on national TV an actual photographic image of Bush in the cross-hairs subtitled "Snipers Wanted" -- no action was taken against Kilborne.)
But since your objection has been framed in terms of individual likeness, ie. that the image in question must be "generic" to be legal for display, what would be the legal status of a virtual "generic" image of a black man being hanged by figures in white hoods? That image would not constitute a threat toward any "actual" black man, nor would its creation require an actual lynching. Therefore, it would not be subject to criminal prosecution, right? Can it be said that it would be any different from a "generic" image that depicted a similarly illegal act against a "virtual" child? Both of these images depict crimes and both of them are bound to offend a broad segment of society, but according to this SC ruling just handed down, aren't both of them legal to display, even if they are presented in an approving context?