Here's the formula...
You have to replace what's between the quotes in the example with the 'location url' for the image you want to post.
To get a location url, just right click on an image you'd like to post and select from the pop-up menu 'Properties'. Then, highlight, copy and paste the location url into the above formula. You can also right click on it and save it to your computer (in case the image is one-day removed from the original source). However, if you want to post it somewhere, you'd have to first upload it to a site like photobucket.com .
More info here:
http://www.davesite.com/webstation/html/tag_ref.shtml