To note, of course you can link to an image on a remote site such as photobucket or imageshack. This is typically how forums are set to work for images.
To explain in a
bit of detail, wrap the URL of the image in img tags.
To explain in a
lot of detail:
Given the URL to the image, such as from imageshack or photobucket or the like. In this case from one of my places,
http://www.feraldreams.com/images/feraldreams_frontpage_html_smartbutton2.gif
You can display the image inline like so:

Which is simply,
[img]http://www.feraldreams.com/images/feraldreams_frontpage_html_smartbutton2.gif[/img]
If you wish to just link to the image: (for instance if it is overly large)
http://www.feraldreams.com/images/feraldreams_frontpage_html_smartbutton2.gifWhich is simply,
[url]http://www.feraldreams.com/images/feraldreams_frontpage_html_smartbutton2.gif[/url]