The image won't display because you've used the URL of the image page rather than that of the image itself, like this:

[img:6b1279bef6]http://img377.imageshack.us/img377/7781/americanflagzf4.jpg[/img:6b1279bef6]

You can achieve this by right-clicking on an image and choosing Copy Image Location (or similar). This will give you the true, direct source of an image without any ad-filled interim nonsense.