Is the image uploaded onto a website? Because if it is just saved onto your computer and not the world wide web it will not work. Once it is on a host site, just right-clich that picture and select properties. Highlight and copy (Ctrl +c ) the URL (I will try an example with the snake at the top of this page). I got this:
http://www.ssnakess.com/forums/header.jpg, now that you have the URL just click the "insert image" picture thing and in the space it gives you hit paste (Ctrl + V ) and it should work. Lets see if it worked for me: