Lightbox has nothing to do with uploads. Lightbox is used under showphoto script hense it is in the showphoto options in admin. Now you can set whatever upload thresholds you want under upload options but they have nothing to do with lightbox settings.
Under your showphoto options if you want to use lightbox both the use big popup window and use lightbox options need to be set to yes. You would then say place 1200 under width and height for lightbox thresholds and save
You can then view a photo and see lightbox I would think.