I was at a reunion over the weekend (details towards end of last week’s MMP).
Lots of pix taken by individuals.
Query: is there some way that each of us can upload our pix so others can see, and if someone wants to download a pic that someone else took, they can do so?
Does Imgur or one of the other picture places have this kind of group feature?
Bear in mind that the level of tech-savvy is going to be low. 
Does everyone need to be able to upload them to the same gallery? Imgur can’t do that. With imgur you’d all upload your own pictures to your own accounts, share the galleries with each other than download whatever you want.
You might be better off setting up a (private) facebook group and everyone can join the group and upload their pictures there. Setting up a “[school] class of XXXX” facebook group is a pretty common way of handling this. Plus when you’re ready to organize the next reunion, or even when you have news for your graduating class, it’s considerably easier to communicate with everyone.
Make a Google Photos shared album that everyone can upload to: https://guidebooks.google.com/iphone/photos/how-to-create-a-shared-album?hl=en&sjid=13367818675749604667-NC
It’s basically a group dropbox.
I agree that Facebook is the way to go. But if you get pushback on that, you can create a shared photo album on Google Photos. Do a search for “shared google photo album” for lots of tutorials.
I would vote for a Google Photos album. If I were in the group and were told that I had to create a Facebook account to share photos, I certainly wouldn’t do it.
Yep- that’s what my son’s Scout troop does. Someone creates a shared album and everyone can drop their photos in there, view them, and download them. Works excellently.
I can’t imagine that Apple doesn’t have something similar, but I wouldn’t know what it is.
They do: Create and join shared albums in Photos on iCloud.com - Apple Support
But anecdotally, we do this every year around the holidays, collecting photos from extended family to make a shared photo calendar. We used to use Dropbox for this when that was still popular, then switched to Google Photos shared albums once that became available. Worked fine for years, and we have a whole collection of these calendars now. Nobody ever complained, which is about the best thing you could hope for on any sort of collaborative family project.
One year, the person whose turn it was to make the calendar decided to give Apple Photos a try. I groaned to myself and put it off, not wanting to deal with yet another account or Apple’s typically terrible software. Apparently other family members felt the same, because a few days later we got a followup email that went “OK…. we’re switching back to Google Photos, sorry about that” 
There was peace in the world thereafter.
Pragmatically, probably a lot more people have Google accounts already than Apple accounts.
A Facebook group also would’ve been a possibility, especially 5-10 years ago, but these days there are people who don’t use Facebook anymore (like myself). Google Photos was a nice, “neutral” ground — usually the lowest common denominator in any mixed random group of users.
This!
You post yours on FB I’ll never be able to even see them (my experience is that one needs to be logged in to FB to see more than a couple, at most), nor will I be able to upload mine.
One word of caution with using Google Photos, any photos you upload, even to someone else’s folder go against your 15GB (for free) cap even though they’re in someone else’s folder. You post some & want to leave them there for at least a week or two for others to look at, & then you forget about them (or decide you want to leave them because they’re from some annual event) that’s nicking at your storage…until you run out.
How does it work if you don’t have a Google account and want to upload to a Google pix run by someone else?
It doesn’t.
If they created a link to that shared album, then you can VIEW the album (without any account).
But to ADD photos to it, you must have (or create) a Google account.
It works that way for most providers. Very few image hosts allow account-less uploads… too much potential for abuse and porn. The ones that do tend to be ad-infested and spam you with recommended images; they’re more social networks than family-friendly shared album.
Anyhow, in my experience, it’s more likely that most of a group of individuals will have Google accounts than Apple/Facebook/Dropbox/etc, because Gmail is so popular.
You (as the album creator) can always create a throwaway Google account and then share that username/password with anyone who might want to upload to the album but doesn’t already have a Google account.