From the Lightbox module README:
Remember, the content has to be Full HTML for linked images!
Optional: To show a caption either use the title attribute or put in the second
set of  of the rel attribute.
If you have a set of related images that you would like to group, follow step
one but additionally include a group name between square brackets in the rel
attribute. For example:
No limits to the number of image sets per page or how many images are allowed
in each set. Go nuts!
If you have a set of images that you would like to group together in a
lightbox, but only wish for one of these images to be visible on your page, you
can assign the “lightbox_hide_image” class to hide the additional images. For
This is very similar to the grouping functionality described above. The only
difference is that “rel” attribute should be set to “lightshow” instead of
“lightbox”. Using the same example as above, we could launch the images in a
slideshow by doing: