Inténtalo tú mismo - JavaScript Reference onmouseenter

<!DOCTYPE html>
<html>
<body>

<img onmouseenter="bigImg(this)" onmouseleave="normalImg(this)" border="0" src="smiley.gif" alt="Smiley" width="32" height="32">

<p>The function bigImg() is triggered when the user moves the mouse pointer onto the image.</p>
<p>The function normalImg() is triggered when the mouse pointer is moved out of the image.</p>

<script>
function bigImg(x) {
   x.style.height = "64px";
   x.style.width = "64px";
}

function normalImg(x) {
   x.style.height = "32px";
   x.style.width = "32px";
}
</script>

</body>
</html>