if the portal was FREE, meaning people won't be charged money by going to the portal, i'm sure there will be willing photographers supplying their images.
the problem now is, money is involved. if you are charging, then anybody whose works appear on the site ought to be given a token royalty fee for having the photo displayed, even if credit is given, but so what ?
of course, this royalty is a very small percentage of each members' subscription fee, but if you were to blossom to 1 million members, then the amount of money involved could be different.
of course, that said, there will still be people willing to provide their photos in return for some form of advertising or something. all the best in your search.