picasa, being owned by google, doesnt really need an app per se.
google is just damn good at making mobile websites. none of their software have apps except for things made specifically for apps (shopper, etc), things that wouldn't work well with the current mobile web technologies (finance), or voice, which really does need an app to be more closely integrated with texting/calling, and for it to be able to notify you of new messages.
all of google's services, like reader and picasa, and so many more, just stay online. Google is a very very big "cloud" company. cloud computing is the whole basis of android, and the upcoming chrome os.