Clive--Sounds like you've done everything right so far. When you upload the banner and save, next to the save button is a preview button. click it and at the top of the page you should see the banner in the current signature box. If you don't then something is wrong. Should only have to do this once unless you want to change banners. It
has to be no larger than 310X90 pixels to work. If its bigger then is just won't save. Making it clickable to your store is another step. I struggled through and can't be of much help, however, Jewel effects has a post somewhere explaining that step by step. I'll see if I can find it and let you know.
Think I found it. If this works, it should help a lot. Jewell (Jan) knows what she's doing.
http://www.jeweleffects.com/createabanner.html