5. I already have an account at Flickr for the quick photos I don't really want to use my own space for. I've set up a Flickr badge (left) to grab public photos tagged with "Library". These aren't my photos, I'm just interested to see what kinds of photos end up with that tag!
I find flickr to be mostly handy, though the login process could be a little more inituitve. I mostly like the streams and sharing aspects of the service. A group of friends can all pool their photos from an event, or a group of strangers can share their dog photos or whatever.
6. Playing with the Flickr tools is timewasting fun, and I think my favourite is Retrievr. You make a simple sketch in a box and it pulls matching images from flickr. It's not really Firefox compatable though, but you can't have everything.
Some of the matches are a little off, but it's still interesting and mildly addictive!