Removed empty src attribute from img tag. Fixes #276

The empty src attribute triggers strange behavior in IE 8 and is a bad practice in general.
If lightbox is used on http://example.com/product/123 IE 8 makes a request to http://example.com/product/ to fetch the image. This caused Exceptions invisible in the browser that we needed half a day to track down.
Chrome and Safari (at least in older versions) try to load the image from http://example.com/product/123, so this can double the traffic on your website in the worst case.
See also http://www.nczonline.net/blog/2009/11/30/empty-image-src-can-destroy-your-site/