This is a community for art in any medium. Welcome!
Rules:
Deviantart. You can also browse Art auction websites. I bet the Met, MOMA, Louve etc all have sites as well. If a site prevents you from copy pasting or right clicking you can disable that with a config flag in Firefox so that you can right click. Or you can open dev tools and get the url and save it there.
Deviant art will let you do everything in one place but it’s not gonna be art from anything more than the age of the internet.
I can think of a few methods that could be integrated together, but no single silver bullet / database / etc.
For example, ArtStation, Bluesky, DeviantArt and Old Reddit all have RSS feeds. If the direct image link gets passed on the syndication, you could potentially set your instance to load the image from the link (if it doesn't automatically) and store it locally. Or maybe even get a robot to download stuff as they're detected.
Pinterest, though as miserable as it is since over a decade to browse, is a great resource for digital or digitalized images, but requires manual browsing. Possibly you can find a lot of older stuff on social media limbos like Facebook and Discord groups and Instagram profiles too.
A bit closer to a database, Booru projects could be useful, though beware as the bigger instances usually seem to be NSFW. But people that post on Booru sites, from what I can tell, usually seem to include the sources of the images. And though by default manual browsing and cataloguing may be needed, projects like RSS-Bridge should give you syndication so you can check where you'd prefer. Twitter should have quite a few artists too, and the Nitter project offers RSS for it. And dunno how Pixelfed behaves towards Mastodon-like microblogging, but if you can follow such and you can set an RSS Parrot or Friendica instance, you could follow these feeds as users in your private instance.
And back on Instagram, the only 3rd party interface I found that seems to be maintained, though not open source, is https://imginn.com/, in case you want to browse there but don't have an account or doesn't want to feed Meta too much information.
If you subscribe to any artists on Patreon, you can download their posts with Patreon-dl GUI. Similarly, if you follow anyone on Pixiv, you can download the images from their posts (just them though, and not the whole posts) with Px Downloader.
If you find an image without source and/or in a poor quality, you can also look for the original using SauceNAO and extensions like Search by Image.
Also there are several artists and art-oriented groups in the fediverse too. At least on Mastodon-aligned platforms, which are the ones I have experience with, you can follow individual users, as well (at least specific to Mastodon) as tags, whose Lemmy/PieFed community handles before the @ get passed as tags there (e.g. posts from here and the community you cross-posted on get passed respectively with the tags #art and #opensource).
Can't think of other specific cases for now, though I hope it helps!
Oh, right, non-fediverse forums may be a good call too.
From the top of my head, for example, GOG's general forum has an old but ongoing thread of fanmade cover arts for games (link).
Or, though lacking the familarity since I don't frequent there as often, the Fuwanovel forum might be a good call too since they're focused on visual novels, a game-like medium where art usually matters a lot.