...ᴀ ғιɴᴇ τʀᴀᴅᴇʀ.

+rep good trader ↔

and take the screenshot with Printscreen ans copy it in Paint and save it as jpg

here a Link to the tut i used https://gaming.stackexchange.com/questions/25965/how-do-i-upload-old-screenshots-taken-with-traditional-methods-to-steam
you havce to scroll down a bit

It is important that the date of the screenshot file match the date you are writing as filename. It is also important that screenshot is JPG.

In addition you have to create a thumbnail of the screenshot and save it in thumbnails folder. You have to use the same name but the width of the image should always be 200px.

Once you have load all screenshot you have to exit Steam and restart it. It could take a while for you screenshot to be shown on the game library, but you can accelerate the process by launching manually the Screenshot management

hello Games without screenshot funktion it gives a way
Go to Steam user data folder located in by default in
•C:\Program Files (x86)\Steam\userdata[xxxxxxxxx]\760\remote\ (for 64bit machines)
•C:\Program Files\Steam\userdata[xxxxxxxxx]>\760\remote\ (for 32bit machines)

[xxxxxxxxx] depend on your Steam ID. 760 is the App ID for Steam Screenshot app.

Now you have to find the Steam App ID of the game you want to upload a screeshot. The easiest way is search the game in Steam Store (http://store.steampowered.com/) and look at the number in browser URL address (e.g. http://store.steampowered.com/app/265610/) .

If there is no folder already with the same name as App ID you can create it using these structure.
userdata
+--[xxxxxxxxx]
+--760
+--remote
+--265610
+--screenshots
+--thumbnails
So now that you have create the folder structure you just need to save the screeenshot in screenshots folder using this format

YYYY-mm-dd_0000n.jpg