Actually, it does do a few things even if Karr is not in the photo.
The infant nephew would verify that the photo was taken around the time in question. The presence of Karr's kids *in Atlanta* means that they went to see other family members which would add to the number of potential witnesses that could testify that they remember the trip and saw Karr in Atlanta on that Christmas day.
If they had chosen to spend a quiet Christmas at home in Alabama, just him and his family, there would be no other adult to testify to his being home other than his wife who, the prosecution could argue, might have a faulty memory or is trying to protect the accused. If there are three or four family members who recall Karr being in Atlanta that Christmas, it will be much harder for the prosecution to refute.
Keep in mind, too, that it is the prosecution that has to prove Karr was in Boulder, not the defense that has to prove he was not.
Even if they establish that Karr was far enough away from Boulder that he had to fly to get there for Christmas night, his presence on a commercial flight manifest would surely have to come next or, perhaps, proof that he rented a motel room, etc.
I know when I see my sister's family, I'm always taking pictures because of my nieces and nephew. But, when I visit my other sister's family, I often don't take pictures because they don't like having their picture taken. Some family dynamics are just that way.
I do hope the Karrs made a copy of the photo or found the negative before turning it over to BPD. Exculpatory evidence like that has a way of disappearing sometimes when a prosecutor is desperate for a conviction.
Whether he was in Boulder the night in question is key, obviously.
But if that's in doubt, the prosecutors would have to establish a nexus between Karr and Ramsey. Have you heard any thing about that? You just don't live in Alabama and pick up and fly to Colorado to randomly murder a child.
He had sons, so he probably wasn't on the beauty pageant circuit, he never lived in Colorado. How the heck did he know this kid even existed?