Hit your reply button to open the "Reply to Topic" screen...scroll down to the bottom of the page where you'll see it says "UBB Code Enabled" click that link and it'll take you to a page with all the various posting instructions...
For those who are wondering - here's another possibility .. assuming the site isn't blocking linking:
Make sure you're actually linking to a picture and not a .htm or .html page. Some sites put their images up by themselves in a way that it appears you're looking at the image itself but you're actually looking at a web page with but one image on it.
Look at the url - it should end with an image tag. If it doesn't then right click on the image and use the view image option. Then use that image url to add it to your post.
KM - exactly why some of those sites make it impossible or difficult to hotlink. Sometimes its not even about "stealing bandwidth" but also about controlling copyright.