Board Thread:Naruto Discussions/@comment-26320097-20150423120614/@comment-24357950-20150423171236

I would guess that if Sakura is the biological mother, Kishimoto is trying to make a joke out of the fact that many people noticed that Sarada does in fact look like Karin. The picture of Team Taka would then be just a normal picture Sakura spontaneously decided to use because she doesn't have another photo of Sasuke or something like that.

If Karin is the biological mother, I would guess that Karin was formerly in a relationship with Sasuke but then died, or that she was in a relationship with him at some point and didn't see herself in the position to raise her child, which would make sense, since she's basically a criminal with no real home village.