Joel,
When you post (or edit your post) the message window has an "Add Attachment" link. Click it and then you can browse to the image on your computer. Images must be 200Kb or less, otherwise they will not upload. 640x480 will usually satisfy the 200kb requirement.
The other option is to just post a link to the online ad in the body of your message here. Good luck.
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Clicking on the banner will take you to the sponsor's website.