You could submit a request to the developers forum for a suggestion of a plug-in for the 'lightbox' option.
With the 'lightbox' option being as popular as they are, it'd be a nuisance having to do that over an over; especially if you've lots of images.
URL to Windows Live Writer developer forum:
http://social.msdn.microsoft.com/Forums/en-US/wlwdev/threads?sort=firstpostdesc
I hope this helps...
Thanks,
L. Henry Jr.