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There's two things you need to do:
1. Go to Admin>System>Banners
To get rid of the exisiting banners you can either edit the settings so that the number of impressions equals the current number for that banner or, if you don't want to keep them, delete them.
2. On the same page, use the "Add New Banner" function.
All you need is the URL where your jpeg sits on the server and, if you want, a url where the viewer would go if s/he clicks on your banner.
I really is as easy as that. No need for any HTML code if all you want is for your jpeg to appear on-screen.
