Well, did you try to upload it on the server? If yes it's only allowed to full members (60 posts) and above.

1. Please use an image share online server (or your own domain if possible) to host your image. I recommend imgur.com
2. Then simply use the direct link to that image.

Sorry to say but Killswitch Engage seem to be using some demon-like images for their covers that can be offensive to some people, so I would be grateful if you could please use something else as your avatar (at least something non-two-horns maybe..).