Anything for a fellow yoshi! First upload your banner to a host.
Second, use the HTML code <img src="adress of the file">
Third, replace adress of the file with the adress. It will be something like
http://www.geocities.com/SuperSonicYoshi/Banner1.png giving you
<img src="http://www.geocities.com/SuperSonicYoshi/Banner1.png">
Fourth, use the checkbox that says "Check here to use HTML in your message" after you are certain that the checkmark is there post the message.
Rori Shangrula, Heaven on earth, utopia, is it hidden miles away, or is it tucked behind your shoe closet... You make the call