You are copying the url of the page instead of the picture to begin with. Different browsers let you retrieve the url of the picture in different ways. For example, Firefox lets me right click and choose copy image location to copy the url of the pic. In Internet Explorer, I right click and select properties. Then I copy the url of the pic listed in the properties.
In Flickr, click the picture and click the size you want. This site will automatically reduce large pics to a more convenient size but allow viewing the size you choose by clicking it. Then copy the picture url as above. Finally, click the Img button above the reply box here and paste the url you copied into the popup box.
