Flickr, probably. I use Picasa (and have a PC), but have never bothered to use Picasa Web Folders (or whatever it's called).. Also tried Photobox (photobox.co.uk was what I actually used), which I liked very much, could share with loads of friends, even those who don't like the Internet much seemed comfortable with it. And tried Picnik, which never seems to work for me (slow connection or slow computer, I do not know).