How to prevent fancybox opening

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jQuery

Hello All,

I am using Jqery Fancybox on my page. I have preivew and upload image button in gridview. I am uploading images using upload image button. If image is uploaded user can see that image on clicking preview button. If image is uploaded iam opening image in fancy box. If image is not uploaded im showing alert message. Problem is alert is showing at the same time fancybox also showing if image is not uploaded. I dont want to show fancybox if image is not uploaded. Please help me on preventing fancybox if image is not uploaded. Please let me know if i am not clear.

Below is my javascript.

function fnOpenPreview(ImgName) {
            var hdnImagepath =                       document.getElementById("ContentPlaceHolder1_hdnImagepath");
            var pnlPreview = document.getElementById("pnlPreview");
            var img = document.getElementById("ContentPlaceHolder1_imgPreview");
            if (ImgName != "") {
                img.src = hdnImagepath.value + ImgName;
                $('.testdata').fancybox
                (
                {
                    'scrolling': 'no',
                    'href': '#pnlPreview'
                }
                );
                return false;
            }
            else {
                img.src = "";
                alert("No Preview avaiable.");
                return false;
            }
        }

Thank you...