Your banner is a ".png" file. I don't have any trouble seeing it, but some browsers may have trouble displaying that kind of file. Convert it to a more common GIF or JPG file to make it easier for all browsers to display it. A GIF will be about the same size, but a JPG will be about half that size, and you won't be able to see any difference between them.
If you don't have anything to do the conversion, try this link to IrfanView.com. It's a good free graphic editor that will allow you to open the file and save it off as a GIF or JPG.
