Clauses like that are really really hard. CC has the NC clause, which is problematic because it may not be clear what consists of commercial use and what doesn't. If a non-profit uses a picture for a fundraiser, is that commercial or not?

The thing that one of the parent comments suggested would actually be quite cool: An alternative license where the "applicants" could ask for permission to use a picture, stating how they will use it. If the author accepts (using a 1-button click), a license will automatically be granted. Everything would still remain free though. And certain use-cases could be allowed automatically (e.g. "personal use" or "use for a non-profit").

It would remove ambiguities (because an explicit permission is granted), but everyting would still remain royalty-free. Plus, the authors would learn how their photos are being used. (I'm happy to give out photos for free, but I'm also happy if people tell me where they're being used.)