I think it's good to have the option to add a picture for indicating differences, watermarks or details.
But I don't support the way of adding it as a second picture.
First picture should show the front side, second picture should show the back side.
As stamps don't have back sides it doesn't make sense in my opinion to add a second picture.
All categories have cases where an additional picture would be useful to make certain things more clear.
However, in my opinion, such pictures should have a special role. (E.g. not visible in lists by default)
As coins, phonecards and other collectables often have cases, too, where clarification would help I don't think we should add there a third picture but some other picture options.
Just my opinion.