Mofo!
Creo que te serviría esto:
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<html>
<head>
<title>fondos</title>
</head>
<body id="pepe">
<script>
var x=screen.width
if (x==800){pepe.style.background='url(imagen1.jpg)'}
if (x==1024){pepe.style.background='url(imagen2.jpg)' }
if (x==640){pepe.style.background='url(imagen3.jpg)'}
</script>
</body>
</html>
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Si tienes cualquier duda... ya sabes... a preguntar!!
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