Like Alan said, currently there is none and there can't be one.
Unsplash has no limitations, most likely authors do not sell those images - or use that site as (free?) advertising medium.
Flicker has no limitations (or I have not seen any; I haven't almost never browsed it).
Stock image sites use extensive semi-random watermarking - this is not suitable for photo galleries.
Some posters on some sites add nice frames and signatures to images - this doesn't avoid downloading, but some work is needed to "clean up" such image.
Hidden watermarks (steganography) are possible, but those can only be extracted and checked from unprocessed (or not heavily processed) digital images - no one will try to find them on t-shirts or on other printed media.
We could in future make portfolios accessible only by logged-in members (optional of course) - but this does not help against someone creating account, downloading images and using them in other places - you even won't know that.
We could exclude images from web scanning by search engines - but this is only hint and won't help against direct download.
