+1 on a Yura chapter being a win! She's grown on me a lot since her introduction as the blowgun club president, and her early appearances teasing Rize and friends, and now I look forward to her chapters.
It wouldn't be the first time we've seen Koi use photographic reference art for inspiration for backgrounds, BUT I do believe it's the first time we've seen the reference art appear in an actual panel of the story, as opposed to supplemental notes at the end of a volume or chapter!
I have to imagine the photo was accidentally left in, in the rush to publish before the hiatus for the next volume's preparation....
I think you're right about Yura - I thought she might have been one of the first characters to mention it, but I was mistaken, it seems Rize might have mentioned it instead as far back as Vol. 10, Chapter 126 in an off-hand remark, with only allusions from other characters since, that I can remember. Then again, it's maybe not surprising: Yura can tease the other characters like nobody's business, but seems to actually be pretty self-conscious, and rarely steps out of the mischievous character she usually plays to say anything serious about herself. So, it's pretty interesting when she actually does do that and we actually find a little bit about who she really is!
And, I do believe Syaro just straight up invited Rize to the prom - that certainly wasn't on my Bingo card for this chapter!
Also, another one for those of us looking for these odd ambiguous remarks: Rize says she feels like she's sending Megu and Elu out to get married... a comment like that is unsurprising at this point where Megu and Maya are concerned (we expect the two of them to be posed like a married couple!), but is interesting where Elu and Natsuke are concerned, especially in light of Rize's curious Sis-Con" joke a few panels later when Elu declares her twin sister unbearably cute! A Jinja twin "sister complex" gag wasn't on that Bingo card, either!