It's a 99% chance that if you're playing the game you're already using steam. Steam is the best file host.
1. F12 to take screenshot,
2. Shift + tab (while still in-game) to bring up overlay.
3. Choose what to upload. Click on 'upload'.
4. Done - your images are hosted on the web - no need to use any other file host.
Now, to post here you need to:
1. Open steam client.
2. Click <your name> -> content -> screenshots
3. Click on the image you need.
4. In the pop-up window right click, then choose 'Copy link address'.
5. Paste into your post.
10. ???
11. Profit