Instructions
Firstly the picture you want must be online somewhere, so if it's only on your computer you will need to upload it to flickr/photobucket etc.
Right click on the picture and (depending on your browser) select: Copy image url.
When preparing your post, look at the boxes underneath the subject, and click the Img button (9th from the left).
What you then see should look like this (but without the spaces): [img ] [ /img]
Put your cursor in between the middle square brackets: [img]I[/img]
Press "Ctrl" and "V" together, and the url you previously copied should appear (but without any spaces):
[img ]http://...............jpg[ /img]