First: upgrade to the most recent Piwigo release, currently 2.10.2

Then disallow registration, and probably easiest is to setup a group and one generic account as member of the group; generic because those can't change the password or other account details. Hand out those account credentials in a thank-you mail. Instead, you could also create a new account for each patron and assign that to the group, to have it more personalized and maybe more fine grained control later, but technically it's unnecessary if all patrons are supposed to see the same content.

If you want to also enable them to upload content (wasn't quite clear from your posting) then take a look at [extension by plg] Community.

Go to Admin -> Albums -> Properties, tab Public / Private.
Select all albums in Public; click on first entry, then Shift+End to select all.
Click on » button => all in Private.

Go to Admin -> Users -> Groups.
(Add group.)
Click on Permissions of the group.
In Forbidden click on first entry, then Shift+End to select all.
Click on « button => all in Authorised.

That's it.

Hi roromx, I have been trying to do something similar to you I think. I wasn't able to send you a pm here to ask you how it worked out for you. I think you may have "form email" disabled in the privacy section of your profile here.

Creo que el mío funciona bien. No soy muy bueno en esto, pero es divertido. Te enviaría la URL, pero no quiero publicarla aquí.

DeepL translates:

roromx: Well, what they recommended to me seemed like a good option, not the best, but it does the job.

anae: I think mine works well. I'm not very good at this, but it's fun. I would send you the URL, but I don't want to publish it here.