In short you need to have the images stored on a third party site such as
http://www.photobucket.com or on your own website.
Copy the url of the picture, which will looks something like
Code:
www.yourdomain.com/yourpicture.jpeg
When making you post on this site click the 'Img' button and copy this url into the message box this will wrap the line in image tags and your picture should be displayed.
Code:
[img]www.yourdomain.com/yourpicture.jpeg[/img]
as an example take my signature I used photobucket.... when you upload it, it tells you under the image the url, or you can right click on the picture and select properties then it tells you the url, you can highlight that link and select copy, the link for my image is
http://img.photobucket.com/albums/v15/d ... -1copy.jpgso
Code:
[img]http://img.photobucket.com/albums/v15/daz306td/sigs/Untitled-1copy.jpg[/img]
gives me:

Hope that helps