My post wasn't an answer to your question, but to do so, it would depend on circumstances.
If such a photo exists, how did it come into his possession ?
If linked to his camera - ( there must have been a camera at some point) - then CB is clearly and intimately involved.
If, however, it was downloaded from some other source, as were many of his pornographic collection, then that is not evidence of his involvement.
I don't see that McCann can be automatically excluded from either scenario, without resolution of exactly how and when MM was taken from the apartment.