Is there iron clad physical proof that *I* couldn't have killed her? Disproving the negative is always a difficult task.
Your benchmark should be "what can the DA prove in court" and, after establishing what can be proven, "is there enough proof that a reasonable jury would convict?"
If Karr managed to kill her and waltz away without leaving any witnesses, footprints, fingerprints or DNA then he probably gets away with committing the crime. You almost can't LUCK your way into leaving no evidence, particularly in a violent struggle.
If this was an intruder, he would had to have been such a pro as to wipe down all sorts of items used during the crime, vacuumed up all the hair and skin cells shed, altered their handwriting so as not to match the ransom note, etc and vanish all without being detected. MAYBE he can do that if he kept gloves and a ski mask on at all times but, otherwise, most people just aren't capable of being so neat.
Which leads me back to the likelihood that either someone already familiar to the Ramseys (and thus has an alibi for leaving trace evidence in the home) did it or that the Ramseys covered up the crime before calling police.
You forget that David Westerfield left no physical evidence in the Van Damm home. And he walzted in past their dog also. It was hair/fibers in his dryer and the print in his motorhome he was convicted on, not anything found in the victim's home. And his was a spur of the moment crime, not well planned.
My point in finding some physical evidence like a photo to prove his alibi (I wasn't very clear) is to deny him the speculation and additional fame he will receive because even though there is no proof he did it, there is also no absolute proof he didn't.
I don't want the scumbag capitalizing on that unproven negative. Because he will and it chaps my behind. I resent this puke being able to sell his filthy obsession (and he can factually state his alibi is only circumstantial and his family has financial reasons to now go along with this, so no more pictures will surface).
If his alibi/innocence was proven rather than circumstantial, then the media would be forced to dismiss him for the perverted crank he is, but since questions can reasonably be raised, they gonna help him make a buck and take their cut.