Picasa is an “iPhoto clone” according to David Pogue. Why not use iPhoto?
Like PupnTaco said, why not use iPhoto? If you’re needing more, consider paying for an app. Aperture is a pretty darned sweet upgrade from iPhoto.
What exactly do you want the app to do? If you just want to view and have sideshows Phoenix Slides is pretty awesome.
There are online photo editors and storage places. http://Flickr.com, http://Photobucket.com, http://iStock.com are a few storing places.
Pixenate is an online photo editor. FotoFlexer is one as well.
Do a search for what you are looking for in the great Google.