Moš javascriptom.
<html>
<body>
<script>
function test(boks)
{
boks.style.backgroundImage=""
}
</script>
<input type="text" style="background-image:url(fds.bmp)" onkeyup="test(this)" />
</body>
</html>
Kad bolje razmislim moš i Css-om zapravo:
<html>
<head>
<style>
.test
{
background-image: expression(document.getElementById("boks").value?"url('fds.bmp')" : "");
}
</style>
</head>
<body>
<input class="test" type="text" style="" id="boks"/>
</body>
</html>