Although I don't see a relationship between these two, this is a very good pic.
For those who have tried to romance Samara in ME2 and have seen/gotten the kiss scene in ME3 Citadel DLC, I can see this scene happening. Shepard was able to break through Samara's "romantic defenses" [for lack of a better term] and Samara was able to show more emotion to another that she thought she could never do again for as long as she is a justicar. Samara's way of saying "thank you" here shows the type of feelings she has towards Shepard and those who are close to Shepard.
Although Liara might not be fully receptive of this gesture, she does realize what Samara means to Shepard and what Shepard means to herself. Another possible thought is that Liara could be thinking of this gesture as something she experienced with her mother, Matriarch Benezia. As such, it put a smile on Liara's face.
Sorry for the lengthy comment, but these were what popped into my mind when I saw this. Excellent pic!