There are many image hosting sites, like ImageShack, PhotoBucket and Flickr just to name a few.
They are all easy to use, with a little learning and trial and error, and sites like ImageShack are free and don't need registration.
Not sure if that is what you require though.
I run a travel company. I want to have an photo sharing link where clients can upload their own photos. Any suggestions as to the best sites for ease of use and functionality much appreciated.