A lot of online photo albums have a username-password login system. You could set up a guest account on one and give the password out to visitors whom you want to allow to upload photos. I would recommend changing the password often, though.
You should also check to see if the photo album program allows you to limit the amount of data that can be uploaded under a single username. You wouldn’t want a malicious visitor to put you over your disk quota.
The program I use is Gallery.