this usually happens because lightbox gets appended to every image . It would look like this in the source code
<IMG src="https://wordpress.stackexchange.com/questions/66237/your image" rel=lightbox">
You need to add a gallery name to the gallery so that it looks like this
<IMG src="https://wordpress.stackexchange.com/questions/66237/your image" rel="lightbox[gallery1]">