Going from memory here, with IL2 each time you press the 'Print Screen' button on the keyboard, the game will save a screenshot inside it's directory. This screenshot was in a .TGA format last time I played, which needs to be converted before you can post it on our forums.
Converting is pretty easy, just open the TGA screenshot in your graphics app, and resize it to a forum friendly size (probably 1024x768 or less) and then save it as a .JPG format file.
Now with this .JPG file, you can follow the directions on this post, which will get your images to upload and appear in your posts.
Showing and viewing images.