Not a mod, just a community member's opinion.
Generally art communities allow posting stuff you found. If you put a link to their gallery or Patreon that would be ideal. The watermark should get you there 90% of the time anyway.
I don't see any reason you would need their permission to post something you found freely online.
Ah okay. Most communities I was apart of most of the uploaders were the creators themselves promoting their work.
If those uploaders were here, they could tell us if they didn't want that to happen.
As they aren't here (yet, ideally), if you don't, no one else will at all, and in my opinion, that is a negative to both this community and the artists, who might pick up a few patreon subscribers through the links and watermarks.
In my opinion, if there's no watermark, do a reverse image search for some socials, cause that's real basic. Otherwise, if you have links, include them, and if you only have a watermark, transcribe any links in there to get more people clicking.
If you did that, I'd consider it well-sourced and very welcome in any online community.