Got this from a mate who should...

Got this from a mate who should know
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1. He will need a high quality image. It can be made as a bitmap image first and then converted to GIF. If he uses JPG format it could lead to additional problems during resizing. Or he may even try it as bitmap image (takes longer to load).

2. He can use a javascript to auto resize image to fit the window. The image will readjust when the window is resized.