The best and easiest way is to use the glMessenger Plugin that includes an online smilie admin and integrates with the forum plugin. You can then add your own images and emoticon definitions.
Someone recently used it to add 52 images to represent playing cards. You could add a company logo or any other standard image that you wanted to be dynamically replaced with a code like :logo: Add/edit the emoticon codes to meet your needs. They updated smilie library of images will automatically be shown when posting to the forum - just as the 22 smilies are here. There is no limit and you can add multiple emoticons for the same image.
For more information click
here
Geeklog components by PortalParts -- www.portalparts.com