To play devils advocate though, that would mean that you could have an
external design shop do a PSD comp for your team -- and potentially
ahead of time, at that.
Again, I'm fuzzy on if this is right or wrong for the intended scope
of the competition; on one hand, this is a 'startup incubator' of
sorts and that is the exact kind of thing you would choose to do if
you were building a web property in the wild and didn't have local
design talent. On the other, having the finances to lay out that cash
for the service is a bit of an unfair advantage in terms of a tightly-
constrained competition.
Anyway... I think, all things considered, I'm leaning towards what you
advocated earlier:
" Paying for something that is created
specifically for the application rather than just a generally
available service seems different to me"
That seems like a good distinguisher to me (or at least as good as
any!), and would cover stock photography as well (since it wasn't
created specifically for you). Enforcing it would / will be hard, but
we can amend the rules to reflect this and ask that people enquire for
specific use case clarifications I suppose?
..nap