Once you have created your banner, assuming the template you are using has given you that ability, there is a bit of a process to go through to get it working on your site. Make sure that your banner conforms to what the template maker has made available to you. For example, the banner area of this site's template¹  is 468 pixels wide by 60 pixels high. When I created the graphic, that is the size I made it. So, here's what to do:

• Upload the graphic to http://yoursitedomain/images/banners.
• Go to Components ⇒ Banner ⇒ Categories. You have to set this up even if all you have is one banner. I just set it up and call it "Banner."
• Go to Components ⇒ Banner ⇒ Clients. As with categories, this has to be set up. I usually call mine "Banner Client" and give it a ficticious e-mail account.
• Go to Components ⇒ Banner ⇒ Banners. This is where you link to the image you uploaded earlier. Select the Banner Category you created, the Banner Client, and then you should see the image you uploaded in the image drop-down box. Select it and save.
• There's one more thing to do. You have to create a Banner module. Go to Extensions ⇒ Module Manager and create a new module. For module type, select "Banner" in the second column. On the new page, give the banner a "Title." Just to be orginal, I call mine "Banner."     Make sure "Show Title" is "No." Once again, choose the Banner Client and Category. Save.

That should do it!