Choose the image you want to post.
Right-click on it, and select properties.
Copy the URL of the photo from the properties box.
(looks something like this:
http://www.triumphrat.net/albums/album746/aaa.sized.jpg)
When posting, click the "image" button below.
You'll get a pop-up, paste the URL of the photo.
And that's it.

It's easier to do if you have 2 browsers running, and switch from the one you're posting to the one you're taking the photos from.