I don't own stellaris on steam, but I wanted to upload a screenshot for a discussion over on their forums.
I tried to follow some online instructions I found online:

I found my steam screenshots directory: D:\Steam\userdata\<mysteamid>\760\remote
(I installed steam under D:\steam, that's not the default, but it's correct for me)

I found the id for stellaris: 281990

I created 3 folders:
D:\Steam\userdata\<mysteamid>\760\remote\281990
D:\Steam\userdata\<mysteamid>\760\remote\281990\screenshots
D:\Steam\userdata\<mysteamid>\760\remote\281990\screenshots\thumbnails

I renamed my screenshot to 20220117035600_1.jpg and copied it to
D:\Steam\userdata\<mysteamid>\760\remote\281990\screenshots\20220117035600_1.jpg
and
D:\Steam\userdata\<mysteamid>\760\remote\281990\screenshots\thumbnails\20220117035600_1.jpg

But when I go to upload a screenshot in the steam client, it doesn't show stellaris listed as a game with screenshots.

Did steam make some change since the posting of the instructions that makes them not work?
Or did I do something wrong?

no those instructions are correct and just to be sure I tested it at my end [see my test image]

two points ;
1. I actually own Stellaris on Steam which may be the tick box you are missing... I'm happy to upload an image for you but it will have my account id in the post

2. you say you renamed the screenshots? Galaxy takes png images and Steam takes Jpg... just renaming a Png to .Jpg does not actually make it a Jpg so try opening your screenshot in whatever you use and save it as a Jpg

thanks, not worth hassling you over it on a per-screenshot basis.

I had loaded up the png in mspaint and saved it as a jpg. Then I renamed the jpg to their naming format. Forgot to mention that.

I guess they are smart enough to know I didn't buy the game there.

But not smart enough to keep me from posting quesitons to their discussions, lol.

As it turns out, by now I've figured out the answer to the question I was needing a screenshot to ask over on steam, so maybe I'll just ask future questions here instead if they need a screenshot.