You have to use tags, and in-between the tags you enter a url of where your image is.
The tags you use are [img*] [/img*] - You will need to remove the *.
Alternatively there's a button, just above the smiley faces, the furthest to the left. Scrolling over it, it will say, 'Insert Image'. This will automatically make the the tags appear, you just need to paste in the url of where your image is.
Furthermore, the url link usually has .jpg .png .gif at the end
If you are wanting to upload a image, that's different, but very straight forward.
