I would guess that if a pic of an "illegal handgun" is developed and the photo mart person call the police, then the police have to have an address of the person who brought in the photo. A photo by itself, without any distinct identifier of the time, date and address of where the pic was taken and by who it was taken, would not be sufficient probable cause for a search warrant, even from the most liberal judge. The most that could be done at this point is go to the residence of the person possessing the photos and investigate whether the handguns are there, or question the person when he gets the photos or whenever. This would require a voluntary admission to search. A photo of unknown origin is not enough by itself, in my opinion, for a search warrant. There has to be much more facts to base a search warrant on. However, if during questioning the pics owner admits to having illegal weapons in his abode a warrant could probably be obtained.
I dont know the facts on the recent photo mart pic of the guy with the bombs but I bet they picked him up, from his name and address from the photo mart, and got some sort of admission from him then did a search either by consent or by warrant after questioning. Otherwise how would they have known he planned to blow up the school?