here is very abstract steps you should fallow:


here is the initial css class to hide the file upload control, you can avoid this step by invoke the js method below on window.load
Code:
.HideMe{display:none;}
this is the js method for toggling the upload control visibility
Code:
<script>
function ToggleFUpload() {
if(document.getElementById('FileUpload1').style.display == 'none') {
document.getElementById('FileUpload1').style.display ='block';
} else {
document.getElementById('FileUpload1').style.display ='none';
}
}
</script>
this is the html markup:
Code:
<img src='your image location' onclick='ToggleFUpload()' />

<asp:FileUpload ID="FileUpload1" runat="server" CssClass="HideMe" />