there's no particular secret to making a background image. most image creation apps will have predefined sizes for you to work in but any image can be used for a background. If a image is smaller then the screen and is reating as it sound like you're getting jsut set the background via css like this
<STYLE TYPE="text/css">
BODY { background-repeat:no-repeat;background-attachment:fixed;background-position:center center;background-image:url(""

}
</STYLE>
this will center and fix with no repeating.
you can also have a function onLoad to find the size of the users screen and fit the background per that resolution. something like
x = screen.width ; y = screen.height
if ((x >= 800) && (x <= 1028)) { x = 1028 }
if ((y >= 600) && (y <= 780)) { y = 780 }
if ((x >= 640) && (x <= 800)) { x = 800 }
if ((y >= 480) && (y <= 600)) { y = 600 }
thus now having a numeric width and height to set the image per that users settings
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