The last message board I used allowed you to link to a picture on the internet, but not on your own PC. To get the link, right click on the picture, and select properties. That will show you the Address of the picture, which you can copy into the IMG tag.
It is a good idea to preview the message to make sure the image is the correct one...
There is also a problem with copyright - you can't just use someone else's image without permission.
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