At first you need a good graphics program. With good I mean GIMP (https://www.gimp.org/)
And, of course, we need a screenshot!

As a good citizien you already have set your Steam to make uncompresed .png screenshots!
Here you can see the uncompressed version!
But before I continue... Lets see what happens if I simply upload the Steam version of the screenshot?!


So usually, after you closed the game, you go to your screenshot manager and simply upload it!

Now the standard thumbnail should look like this!
YIKES!

So here is what you want to do:
First of locate both screenshots you made. The Steam one and the uncompressed one.
The Steam one is usually always under: "C:\Steam\userdata\ YOUR STEAM ID \760\remote\ GAME ID"

For Atomic Heart (My screenshot) it is this one:
Then I locate the uncompressed one. Standard loaction is your Picture folder (But you can let Steam save it wherever you want!)
Now I open up the uncompressed screenshot with GIMP
IMPORTANT

Now you have to take the name of the OG screenshot of the steam screenshot folder (C:\Steam\userdata\ YOUR STEAM ID \760\remote\ GAME ID):
You need to copy that name!
Now back to GIMP and go to "File -> Export As..."
You rename the file and save it wherever you want (As long as you can find it again that is)
When you press EXPORT make sure to check the following
Set quality to 100, if posssible and also activate "Show preview in image window" to see the final size!
You can't upload screenshots that are larger than 10 MB (9,99MB is the maximum!)
If your shot is below 10 MB, press Export again!
Now you can take the screenshot
And replace it with the one in your Steam folder:
"Well now I can upload it ... right?"
Nope! You now replaced the OG screenshot with the uncompressed one, but you also might have noticed the "thumbnails" folder there as well, right?
In the thumbnail folder you can find the thumnbnail (OMFG this is such a stupid text) Steam would use for your gallery.
I mean.... Yeah... You don't want that!
So we simply overwrite that thumbnail with the edited uncompressed shot we made prior:
"Are we done yet?" you might ask? If you upload that thumbnail you are 100% going to kill your Steam gallery! Everytime someone opens it up Steam would be forced to download the 8,77 MB thumbnail!
It might not sound bad at all, but now imagine you do that with every screenshot!
Steam would have to download 50 times 8,77 MB at once!
This can cause Steam to either lag out, not showing most thumbnails or literally yeet itself into oblivion!
What we do now is take that super huge thumbnail of our shot and open it with GIMP!
Use the "Scale" Tool (if it is not there press right click on the "Unified Transform" Tool and you should be able to choose it!)!
Now click on your Screenshot and a smol window will pop up:
Choose "percent"
Make sure both sides scale equally!
And scale your shot down!

• For 1080p 50% should be enough!
  
• For 4K 33% should be okay!
  
• For HUGE Panoramas 25% is the goal!

Alright:
Because my shot is 4K, I go with 33%
Now it sohuld look like that!
Press "Scale" and wait a bit!
Now go to "Image -> Crop to Content"
Now it should look like that:
Now simply go to "File -> Overwrite SCREENSHOT NAME"
Now you should make sure that your Thumbnail is around 500kB (200 more or less isn't that important!). Just keep it small!
Export!

DONE!
Now you can upload it!
Restart Steam now, so Steam can refresh the Screenshot Manager!

Before:
After: