Tough one?? That worked perfectly!! It’s so simple that not anybody could have come with such solution.
Sadly, the page header with the link to the full image isn’t detected or modified by lightbox plugins, so it keeps opening in a new window.
So, I have taken a couple of days to look for other solutions and finally have decided the easiest and fastest way for me to achieve it (for the record):
– setting a “Featured Image” for the excerpt
– adding CSS founded in this forum to hide that Featured Image in the complete post
– adding the same image along with the normal content of the post
– linking that image with the full size image, it gets “lightboxed” automatically if you have “WP Lightbox 2” or “Lightbox Plus Colorbox” plugins installed and activated (ad probably others, but not all)