Although I'd kill for it being Jesse's I think it's Olivia's
[edit]I thought about it again and the answer is simple.If the finger from the left is a thumb it's Olivia's and if it's the small finger it's jesse.I zoomed in and I think it's the small finger... :)))))
It has to be Jesse's(Chase) hand because Olivia's (13) arm looks like its touching Jennifer's(Cameron) waist section and not right behind her head.Plus, it looks like the last finger is Jesse's pinky not Olivia's left thumb. Also, Jesse's shoulder is inclining upward like he's in the process of putting his arm around Jennifer.