Those pictures are to sell products tho so they will want to show as much of the suspender belt as possible, and obviously they have to wear knickers to be allowed to have the pics online....
So I would ignore those pics in terms of practicality.... seems sensible to wear them under, if wearing them during the day you need to be able to go to the toilet, if wearing for sex, you need to be able to get your knickers off ;)
I'm an under....