I hadn’t realized I had turned off this feature (or the default is off and I never changed it). You can now link to images in addition to attaching them. You do this by enclosing the image URL in tags like this:
[img]include.url.here[/img]
Example:
[img]http://www.nvidia.com/docs/CP/38852/GelatoDOF.jpg[/img]
And it comes out like this:
