viewtopic.php?f=27&t=55 shows how to attach local file(s) to a forum post.
Similarly, one can attached an image that resides somewhere else that can be referenced by an URL:
1. From where you want to insert the image, click on the "Img" button that is on top of the text entry box. These are short cuts to BBC codes that is what drives most of the forum sites.
2. Fill in the actual URL in between the 2 IMG open/close tag.
3. You can then submit or preview what you'll get.
NOTE: the one draw back from a live URL reference is just that, it is linked to a live URL file somewhere. So if that URL/file becomes not available, you will get a 'dead end' image instead of the actual image you want to link to. So if it's something that you really wanted to link/show, you might want to consider either downloading that image or screenshot that image and then post up with using a local file attachment instead of using a live link. This is true for any other URL as well (i.e. PDF file, if it's important enough, make a local copy and then attached it).