Capture the image (Print screen, then Paste in a program like Paint). Save the image on your HDD (e.g. D:\Pict.jpg)
Then open the post on the forum and clic on the IMG button. In the text box point to the picture on your HDD (D:\Pict.jpg). Check it using preview. That's it. It should look like this:
Bluesky
Or not X( . The file should be placed on a server.