I forget which online service it was, but back in the dot-com days, a friend stored all of his photos on one of these photo websites. Well, the website went bankrupt without warning and went offline one day, but they eventually offered to return them for a fee. He was not pleased. Definitely read the user agreement if you want to go this way to understand your rights.

I'd say that sites like these are great for sharing photos (don't have to email photo files to a bunch of people) but for long-term storage, better to keep the files under your control.