Note: Motorcycle-Journal now supports image attachment in the advanced post dialog. You don't have to host your photos elsewhere. See the second sticky. If they're already hosted elsewhere, follow this method.
The image has to be hosted on another site. (i.e. Imageshack,etc...yahoo doesn't work because the url is too long.) Once you have done this, bring up the image and right click on it. Click properties and then highlight the url of the image location, right click , copy. (click and drag) Come back here and paste that url between [img] and [/img], or in the message control, click the tiny pic of a mountain and paste the url in the window. Voila.