Nearly there........
Right click on that photo (or any you want - I've used the one you linked to)
Click on properties
Cut the address (Url) the one thats starts
http://img.whateveryouwantbutnotyourharddrive.jpg
Click on picture link in the quick reply message box (the postcard icon)
Paste in the address you cut, into the box that opens
Preview post is a good tip right now
And Del's yer uncle.