I think the girl from his childhood picture is Yotsuba (the ribbon girl) and I have a few reasons to believe that.
The first thing I went back to check on were panels where they were lined up together. While they were not always in the same order, there were a few cases where they were shown in a specific order:
(Last page(excluding credits) of the One-shot/Ch1): The shot of the girls
(Ch8 p.13): Shot of their expressions viewing the fireworks
(Ch21 p.8): Mental image of their backs formed by Uesugi
which leads me to believe that the girls are also in that same order on (Ch14 p.19) in their group shot as kids.
Now why would the order matter? Whether it is intentional or not, I believe it is a hint and how the author mentally sorts the girls.
In chapter 1, Yotsuba was the first one to show genuine interest in him and called out to him with familiarity by his name. At this point, Uesugi himself hasn't told any of them his name and when he asked her how she knew his name, she brings out his test paper which she probably got from Itsuki - a document which has his full name. It's possible that he forgot/never knew her name but she learned/remembered his. Also, no one else addressed him by name before he was their tutor except for her. She was following him around school that day.
In (Ch13 p.17) where she turns down the basketball club invitation, could have been her alluding to spending time with Uesugi rather than studying (he's the tutor anyways). There was also the incident in (Ch 21 p.17-19), when she reacted differently after Uesugi called her "straightforward and honest" because she knew she was lying or hiding her identity from him. She then said that she liked him as to attest to her "honesty" but decided to brush it off as a joke in the end.
If you compare the expressions in his photo to the group photo, the childhood friend seems to be either the one on the far right or the 2nd from the right. The scenes mentioned above have them in this order: Nino, Miku, Itsuki, Yotsuba, Ichika and if the group photo follows suit, it would mean that his childhood friend would be either Yotsuba or Ichika. There's also the similarity in Yotsuba's ribbon and Uesugi's current hairstyle (the sprout ahoge thingy - this part seems more farfetched than the others but I'll include it anyways) which could be a throwback to something they've experienced together as kids. Couple that with her behavior towards Uesugi, makes me believe that she is the girl in his childhood photo. Try a reread with this in mind.
Or maybe I'm looking into it too much into it and she's just a regular genki girl.