Unable To Get Id Of Image Button

I wish to get the id of the <input type="image"> in the client-side before it generate the postback in the server-side. However, i always received a "null" or "[Object]", which is displayed using the "window.alert()" method.

This is my coding used to create the image button:

<input type="Image" name="1.jpg" id="1.jpg" onclick="Btn1_onClick(this)" src=Images/1.jpg class="removeBorder" onmouseover="this.className='applyBorder'"
onmouseout="this.className='removeBorder'" />"

Below is the Btn1_onClick() which is called onclick:

<script>
function Btn_onClick(control){
window.alert(window.document.getElementById(control.id));
//window.alert(window.event.srcElement);
}
</script>

I have also tried using the "window.document.getElementsByTagName(tag)", "window.document.getElementsByName(name)" and "window.event.srcElement", but still return the "[object]" message.

Btw, i am creating a user control (.ascx) in ASP.net, therefore i wont be able to use <form> tags.