Bizarrely enough I was stumped on exactly the same thing for the last couple of days! Working great now thanks to stumbling upon this post.
Just in case anyone followed this
excellent article and then got stuck at the end, your embedded image tag should end up looking like this:
<sfLib:ImageGallery id="ImageGalleryControl" runat="server" GalleryTemplateMode="Lightbox" ItemListTemplatePath="~/Custom/Public/ControlTemplates/Libraries/Images/GalleryLightboxList.ascx" />
Now the lightbox works like it should.