My lightbox script picks data from four different elements.
For the title it grabs (in order of precedence):
1. the link
title attribute (a:title)
2. the (nestled) image
title attribute (that's the a->img:title)
For caption (in order of precedence):
1. the closest
2. the closest
The default twentyeleven theme use these divs to display image captions in galleries and for standalone pictures respectively. If you don't use them, you won't get captions in the lightbox.
We have atleast three fields in the image upload dialogue; title, caption and description. It is your theme that decides which ones are used on the front end. Compare your theme with the default bundled theme if you're unhappy with where the texts show up.
@scruff - your styling is sick! I would be super happy to add that to the plugin. Send me an email: ulf at ulfben dot com.