Open the page on your Flickr for the photo you want to post.
Right click on the picture itself and select either 'Copy image address' if you are using a Mac, or 'Properties' if using a PC.
If using a PC, copy the URL from the box that appears.
Post a message here as normal.
Click on the little yellow picture of the mountain above the box you type in, and then paste the image address in the box that pops up.
Click 'OK'
Click 'Post'
Picture should then appear.