Thanks, I was hoping someone took the time to give them a try. I just stumbled upon them and wondered. Thanks for the information.
If you look at the "related articles" link at the bottom of this page, you will see discussion of SuperStock from 2007 through 2009, then a post about them going bankrupt and owing contributors a ton of money. Then nothing until this thread, and one other that started a month later (in May 2018).
I don't know anything about them myself, but the history of discussion of them here tells me to steer clear...