It's fixed now, but...
Now any images that are classed as 'thumb' are put into the lightbox.
This is working different to 2.3.0, IIRC, in which it was embedded in a frame.
edit:
To clarify: if I use thumbnail, instead of using the thumbnail image, it drops it into the lightbox.
If I don't I can resize the image (not ideal as I'd prefer the thumbnail)