Ok, so I've spent way too much time sifting through all the game's images and some of the dialogue lol I'm not sure if I found anything of note. Here on out, beware of spoilers; I put the relevant photos in their own spoilers too so it wouldn't clog the screen when people scroll. These are all the clues I (and you all) are aware of so far:
1) Halloween photo that started all this of course; Guy found it during halloween update and then had some kind of dream related to it that was apparently very, very important. Later upon questioning, Ashe explains it was taken at the "Hirsch Home"; "It's like uh, a historic building downtown? They rent it out for parties and stuff. The Hirsch family was really involved with the city a few decades ago. Old money types. Made all their money in textiles. Nowadays they mostly keep to themselves, though."
Guy's thoughts about that, btw: "It's not the place. Or the Hirsch family -- though you do mentally file the name away for later."
2) During Ashe's date in the jacuzzi, Guy remembers a bracelet (it appears on Ashe's wrist then disappears for the player's benefit; people thought it was a bug). It's the same bracelet she was wearing in the halloween photo.
Ashe's choker in that halloween photo also looks (almost) exactly the same as Brit's choker she wore as a 'goth chick' during her photo shoot with Guy. That's already been mentioned multiple times in this thread by others, but don't know if that's an actual clue or if it's just a matter of limited art assets available to a game dev lol. Guy himself hasn't referenced it at all I don't think.
Brit also wore the same bracelet, though, but Ashe mentions that the bracelet in her photo was a cheap prop; Guy figures Brit's was too. This could also rule out the choker as the same deal. They're both potentially red herrings.
3) On Ashe's romance route, the morning after her date, she tells Guy that it might not be the photo itself, but it simply reminds him of something. This sparks him to look at the other photos and he lands on one of Cole & Bonnie adjacent to Ashe & whoever she's talking with (who himself may or may not be important?).
Then that sparks another memory where Maya (Carolina's kid) took a photo of him & Nicki (photo was taken before this whole halloween party photo mystery began, I think? I have no recollection of it).
He juxtaposes it with the one of Cole & Bonnie, and... I don't know, thinks it's interesting for some reason? Similar poses & perspectives, but... doesn't do anything for me.
Anyways, that's mostly all I got regarding Ashe's halloween photo. I've looked for poses in other photos that were similar to Ashe's in her halloween one, thinking maybe if it was a photographer that was the link, maybe he (or she?) has some favorite poses he likes to request or something. Nada. I already knew about the hint on Ashe's romance route that Neon mentioned, but that didn't spark anything for me either. Thought maybe it might have to do with w/e conversation Guy & Nicki were having when Maya took that photo, but couldn't even find it. Considering this whole thing deals with Guy's subconscious, I mean, I don't know how abstract or concrete this is all supposed to be. Like, is it something directly related? Indirectly related? Related only on an abstract level if you squint hard enough? 
But if anyone remembers anything else that I missed or forgot, like key dialogue or at least something interesting that could be related, let me know! I'm mostly lost lol Maybe I will just keep my hand around my cock during this mystery where it belongs.
Now, ya'll can skip all the rest of this if you're not interested, as it might not pertain to the halloween photo mystery and there's some definite conclusion jumping on my part, but.. on my quest, I did think about some things, may have found some things, all relevant to the story. It's possible others have beaten me to the punch, but I can't be assed to search... so, first, some things I may have found:
I found this photo from the halloween update of a pregnant Carolina dressed as a robber and, presumably, her man dressed as a cop.
He looks like Lucien to me. Again, this could easily be a case of limited assets being reused.
Here's two crazy theories if it's him: 1, is he Carolina's ex? And Maya's father? Or did he just get a photo with Carolina at a party they were both at? And 2, was he a cop in the past? Or is he still a cop (could be officially undercover)? Obviously people can buy cop uniforms & wear them for halloween. Buuut.. if it's him, and he used to be a cop and so already had a uniform to use... it would conveniently explain his connections with certain corrupt officers. That could also mean his real name might be Michael. This is all probably just way off the mark, though at least it's fun to consider.
I also found this: during the Halloween update, after Guy had spoken to Mr. Price about this halloween-themed calendar, Guy is trying to think of ideas for the shoot. Nicki walks in on him and finds him looking at this image:
And then, upon her advice to use 'real looking girls', he goes searching for inspiration from halloween photos of the girls in his life. Looking at Cassie's, he finds this:
Notice the gloves in that pornographic photo? And then the girl's outfit, to the left of Cassie? Looks like the same girl. And Guy's sister was around that girl at a halloween party? It might even be the same halloween party where that photo was taken; well, maybe not the same building, maybe the photographer picked her up at the party and brought her to his fancy mansion or w/e. The girl in the photo, btw, her eyes look like a deer in headlights. Does she even want to be there, spreading her legs to what could very well be some creep? A creep that is potentially in the vicinity of Cassie? She hasn't been responding to Guy, either... her own volition? Or..?
Now, my thoughts about one of the story's potential themes: this is probably yet another deviation, but because one of Lucien's aliases is the same as the asshole who took nudies of Ashe on a beach, another person in this thread mentioned that Lucien may have been the one who took those photos (not the halloween photos, in case all this photo talk is getting mixed up ). He then pondered if Ashe's dad is the one who gave Lucien his scar. It's interesting and got me wondering if this story is supposed to have themes of cause & effect, interconnectedness, or something? Because Risa said Lucien changed after that, like his personality even. Obviously Cole isn't responsible for what Lucien does, but he could be an indirect cause of all this twisted shit Lucien's doing if Cole beat him bad enough to inflict him with one of those weird brain injuries that make people more violent etc. Bit of a thin thread, but maybe. Neon did say he thinks violence tends to cause more further problems than it solves, wouldn't at all be surprising if that was a core theme. I also happen to agree with him Would tie up this little Lucien bow pretty strikingly, imo, knowing that it didn't have to happen this way; that all these girls suffered because of a fucking fluke injury from an angry dad's beating, albeit perfectly understandable, even perhaps justified.
Whew, ok. Wall is officially complete. To those who took the time to read, thanks lol. I postponed shit I need to do to type all this out
