A Lightbox consists of two images - an image for the thumbnail view and an image for the full-screen view. This is not easily visible in the "Settings (D)" panel, though. Actually it's quite a mess - on the one hand you see "Lightbox settings", but on the other you also have "Replace image" and "Tab settings". The latter does not make much sense as a title in this case :)

It'd be much simpler to have an "Upload thumbnail image" title and an "Upload full-screen image" title.