That doesn't appear to be a valid image address. Imgur gives me a 404 when I try to open it.
Excuse me, I mean it DOES "appear" to be valid because the configuration is proper and it does take me to an Imgur server. But the detail of the URL you provided is not one Imgur can resolve.
To post an image, you should be able to paste that URL into your post, highlight it and click on the IMG button (3rd button in the second row from the top)
The result will be a string that looks like this:
Then you can post it