rav0 code should work, but if it is only an image, here is a simpler way to do it:
code:
<?PHP
$browser= $_SERVER['HTTP_USER_AGENT'];
if(strstr($browser,'MSIE 2') OR strstr($browser,'MSIE 3') OR strstr($browser,'MSIE 4') OR strstr($browser,'MSIE 5') OR strstr($browser,'MSIE 6')) {
$showimage= "<img src=\"/images/ie-image.gif\" />";
}
else {
$showimage= "<img src=\"/images/normal-image.gif\" />";
}
echo $showimage;
?>
Just put that where you want the image to show. Change the image links of course, but leave the backshlashes (\\\\\) as it is important for the syantax.
/ 
