How can people post images to my Message Forum?

Images can be posted using BBCode or HTML code, so long as they are allowed in your Settings (See: How can I turn off HTML and BBCode in my Message Forum?). Example BBCode is below:

[img]http://sample.com/image.gif[/img]

Make sure the url is correct for the image, and make sure the image you are linking to is stored on a webserver that allows you to remotely link to it.