This layout is 600 pixels wide. A banner image automatically has a 15 pixel border in a basic template, this is the green area that you see around the banner. The color of the border is set on the Design > Style page for the template, in the Page Banner element with the "Background" color.
To create a banner that fits perfect, you would subtract the 15 pixels for each side border (30 pixels total) from the width of the layout, creating a banner that is 570 pixels wide. The height will adjust automatically.
November 25, 2003 | Permalink