Yes, that probably could be done with a bit of modification of the lightbox script, probably more than just a little bit. However, I see no reason to. If you want lightbox activated onclick of an image, you may do this:
Incidentally, if you happen to have an image map, lightbox can initialize on an area tag as well:
<a href="some.jpg" rel="lightbox" title="Some Picture"><img
So an href link is not required, but an href link or an href area tag is.
<area rel="lightbox" shape="rect" coords="0, 22, 30, 60" href="some.jpg" title="Some Picture">