<script language="javascript">

var img = new Image();

img.onload = function() {

  var w = this.width;

  var h = this.height;

  alert("width : " + w + ", height : " + h);

}

img.src = '[이미지 경로]';

</script>

<script language="javascript">

var img = new Image();

img.onload = function() {

  var w = this.width;

  var h = this.height;

  alert("width : " + w + ", height : " + h);

}

function setImgSrc() {

  img.src = 'http://i1.daumcdn.net/cfs.tistory/static/top/tistory_logo.gif';

}

</script>

다음 이미지를 클릭해 보세요.

<img src="http://i1.daumcdn.net/cfs.tistory/static/top/tistory_logo.gif"

 onclick="javascript:setImgSrc();" />