On iStock, for a lightbox to be public, it must have more than 20 photos and more than one photog pictures in it.
I have on on Morocco with abuot 30 odd pictures from various people (though mainly myself - I choose morocco as it is the theme with the most photos of mine). I choose other peoples photos who complemented mine rather than direrctly compete.
However, Morocco is not a popular topic so I have only had one view in the past 2 weeks.
I think I have a picture of Roman ruins in someones lightbox and that is my biggest selling photo. If you have a good photo, you can send a light to the owner of a lightbox and they can choose to add if they want - there are a few general ones like 5 star with no sales (they get removed once there is a sale) and first ever download, 5 of 5 for new joiners (upto 5 5 star photos for photgs with less than 500 DL). Any of these general ones can increase exposure but as they are general, seriuos desigers probably dont look at them.
As I get more photos and more themes, i will probably do more lightboxes. I have not used them at other sites.