HOWTO: Adding a watermark to images when correspondent checkboxes are selected

function addUploadFileHtml(index) {
    var guid = imageUploader1.getUploadFileGuid(index) + '';
    var fileName = new String(imageUploader1.getUploadFileName(index));
     /*… Code ... */
    //Add Chackboxes
    h += "<input id=\"checkbox" + guid.replace(/[^0-9abcdefABCDEF]/g, '') + "\" class=\"checkbox\" 
    value=\"a1\"   type=\"checkbox\" align=\"middle\" checked /> <label for=\"checkbox" + 
    guid.replace(/[^0-  9abcdefABCDEF]/g, '') + "\">Watermark</label><br />";
     /*… Code ... */
}

//This function adds the watermark to the images, which are checked off in the upload pane. 
function ImageUploader1_PackageBeforeUpload(packageIndex) {
    var guid = imageUploader1.getUploadFileGuid(1) + '';
    var cb = document.getElementById("checkbox" + guid.replace(/[^0-9abcdefABCDEF]/g, ''));
    if (cb.checked) {
       //Apply watermark 
       imageUploader1.setUploadThumbnail1Watermark("opacity=100;ImageUrl=Watermark.png;
       ImageWidth=155;ImageHeight=30;Position=BottomRight;");
    }
    else
       //Do not apply watermark
       imageUploader1.setUploadThumbnail1Watermark("Position=TopLeft ");
}