Please make sure, that you have Steam Overlay enabled in your game.
1. Go to Steam -> Settings
2. Click the In-Game tab
3. Check "Enable Steam Overlay while in-game"
4. Go to Library
5. Right-click on the game title and select Properties
6. Under the General tab also make sure "Enable Steam Overlay while in-game" is checked
To make a screenshot:
In the game, press F12 to take a screenshot. Then press Shift+Tab to bring up the Steam overlay, and Upload your screenshot, make it public. Then simply include the URL of your screenshot on the forum (it should look something like "http...sharedfiles/filedetails/?id=...")