This method involves the use of an external program that you have to download and execute on your computer. We understand that not all users will be comfortable with this, and suggest that if you don't trust us, try the tool on a seperate computer / virtual machine with a different Steam account or read the source code and compile it yourself.
The tool interacts with Steam using the Steam API, by tricking Steam into thinking a legit screenshot was taken. Because of that, the program is VAC-safe.

Mirror 1: https://drive.google.com/file/d/0BwSFV9LmCqmiVjRGTWluOXdIbmc/view
Mirror 2: http://www6.zippyshare.com/v/dPiz0ZsL/file.html

https://github.com/0x6FA3D0/SteamScreenshotUploaderGUI/

Once you have downloaded the RAR archive, extract all the files to a folder of your choice and run the program.

Fill in the AppID of the game you want to upload the screenshot to and location of your choosing (will be displayed under the screenshot). Optionally, you can include players that you want to tag on your screenshot.

Reminder
You can find what's the AppID of a game by it's store page URL.
For example, CS:GO's store page is http://store.steampowered.com/app/730 and so it's AppID is 730.

You can now select the image you want to use and hit upload.

Troubleshooting
If you got an error, make sure that

• You own the game you're trying to upload to.
  
• Steam is running.
  
• You extracted all the files from the archive, and they are all in the same folder.

If the upload succeeded, the screenshot should appear right away in your Steam's screenshot manager. Simply find it, give it a name, and upload!

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