Sadly I don't think the anime will tease this ship any longer... Or with Riko, but wow I enjoyed the ride so far, You is such a great character, without a doubt my favorite LL girl.
But if they make a mooooovieeeeee uuuuuuuuuuhhhhh BOI
The description in the Pixiv link confirms that it's Riko. Also says something along the lines of the photo somehow getting out to the other members, and poor You-chan getting really embarrassed about it.