To make the screenshot just press the "Print Screen" key (usually located above the numpad or the arrow keys, depending on the keyboard model). You can make one both during the game, or the end-game score table.
2. Locate the screenshot
When this is done, Your screenshot should be saved as a randomally named JPEG file in Your WarRock directory. On my computer it's D:\Games\Warrock\, but by default instalation You get something like C:\Program Files\K2\Warrock\ (or so).
3. Upload it to some free image hoster
After You found the screenshot and remembered it's name (You can also copy it to some other, newly created directory to avoid mistakes), You're ready to upload it to some free image hoster. I'd suggest using imageshack.us as it produces perfect BBCode for this forum software.
On this website just click "Browse...", locate Your screenshot again and then click the red "host it!" button.
4. Copy thumbnail code and paste it in forum
The final step. Copy (CTRL+C) the field called "Thumbnail for forums (1)" and paste it (CTRL+V) in Your new post editing form!
That's it!
Hope You find this helpful.
Cheers!
:>