I'm gonna assume you're talking about In-Game screenshots.
I'll try to explain it to you. You wanna go to your profile then click the "screenshots" button
Then you should see this screen
Once you see this click "Last session" then select the game you wanna upload the screenshot from
I've taken Anno 1404 as an example. Highlight the screenshot you want by clicking it once, then you press "Show on disk"
Show on Disk
Once you've clicked this button you'll be taken to the screenshot folder, where you once again have to find the image which you can then drag to your desktop or any easily accessible location then upload it to imgur
If you weren't talking about steam screenshots, then try to explain what you want more clearly so I can help you with that.