There are few solutions to this situation, you can read about them here. The easiest one is to declare two javascript globals to both gifs in your _Layout.cs:
<script type="text/javascript">Then your config section in lightbox.js and lightbox-web.js should start like this:
@*Global paths Lighbox Images*@
var fileLoadingPath = '@Url.Content("~/Content/themes/lightbox/loading.gif")';
var fileBottomNavClosePath = '@Url.Content("~/Content/themes/lightbox/closelabel.gif")';
</script>
LightboxOptions = Object.extend({
fileLoadingImage: fileLoadingPath,
fileBottomNavCloseImage: fileBottomNavClosePath,