Hi Seamore
As others have said the easiest way might be to upload it to your personal album (linked from the left hand main menu under 'my AditNow.co.uk'.
If for any reason you can't do that (copyright restrictions for example, or just don't want to upload somebody else's image) then grab the actual internet address of the image (you can get this by right clicking in Internet Explorer on the image and looking at the 'Address URL' value.
Paste this into the forum message like this:
http://www.aditnow.co.uk/images/misc/merchandise.gif (that's the little t-shirt graphic from the site home page)
And then mark the text in the message (as if applying a style like Bold / Italic etc in Word or such like) if you've got it right the text will appear white on dark blue.
Then click the [Img] button and you'll get something like this appear either side:
[ img ]http://www.aditnow.co.uk/images/misc/merchandise.gif[ /img ]
And when your message is posted it'll appear like this :)
my orders are to sit here and watch the world go by