The download contains a standalone uploader
I setup a Picasa account to try it out and it seems straightforward enough. The advantage Zooomr and Flickr have over Picasa is how easy they make it to include pictures from either app in your blog posts. Picasa doesn’t make this function available (or I couldn’t find it).
Having both Zooomr and Flickr accounts, I don’t see any advantage to having a Picasa one as well. I suppose if you had a Picasa account and recently got a Mac, then this is useful. Otherwise (unless I’m missing something rad in Picasa), use Flickr or Zooomr.