<SCRIPT LANGUAGE="javascript">
if (screen.width < 639)
{document.write("Hello there")}
if (screen.width == 640)
{document.write("<IMG SRC=630px.gif>")}
if (screen.width == 800)
{document.write("<IMG SRC=750px.gif>")}
if (screen.width >= 1024)
{document.write("<IMG SRC=850px.gif>")}
</SCRIPT>