• for this to work you will need a more recent version of steam that has a modern screenshot manager
  
• from steam go to View > if at the bottom it says Recordings & Screenshots - you're good
  the previous Screenshot Manager also works but the old one may/may not work

1. IF you haven't taken screenshot of the relevant game via steam client, take random screenshot (f12) -- even the opening splashscreen will do
   
   
2. from Steam Client go to View > Recordings & Screenshots (previously Screenshot Manager)
   
   
3. click the Show on Disk
   
   
4. Copy your image file to this folder
   Note!! make sure your file format is jpg, not JPEG/PNG/PSD,etc
   
   
5. restart steam client (Steam>Exit)
   or press windows+r and paste this
   steam://exit
   
   
6. when you open steam client again and go to Recordings & Screenshot, you should see your custom screenshot. You can just right click on it and share on Steam as usual.

If you are still using older steam and don't want to update, you can use 3rd party tool to upload custom screenshot
https://steascree.download/
i will not be covering the process here, but it's quite a straightforward process, though it's not *cough* 'instant' like this one, hehe

if your screenshot manager bugs out, ie. it's not showing your local screenshot (only showing the ones from the cloud)

• exit steam
  
• navigate to your steam folder
  ie. C:\Program Files (x86)\Steam\userdata\[your id]\760
  
• delete screenshots.vdf file
  
• re open steam, and it will auto repopulate your screenshots

don't know what cause this bug, and it very rarely happened if any..

Thanks a ton for reading — hope this guide helped you out! 😊
I'm still learning the ropes when it comes to both writing and the process itself, so feedback (tips, critiques, wild rants) is more than welcome! 👍

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