Goals

PicaSto has the following goals:

Separation of content and presentation. Because the metadata is published independently of the albums and photos in an open format, a client is free to present the photos with whichever interface it chooses, therefore enabling the user to view photos from different sources in a uniform and user-chosen way.
Decentralization. Photos can be published anywhere the publisher has uploading access to, so the publisher doesn't have to entrust her photos to one specific authority. The photos can even be published in a completely different location from the metadata, for example on a dedicated picture host, while the comparatively small metadata files can be published on the publisher's homepage.
Easy publishing of pictures on a web server. The metadata files are just plain XML files, which are served by a web server along with the photos. All the logic can be shifted to the publisher and the client, which imposes no additional requirements on the publishing location.
Integration with existing systems. The publisher doesn't have to stop creating "traditional" HTML galleries for her photos, but can just provide the metadata files as an additional service for consumers.