Since Photostream and Shared Albums were mentioned above, I would add this caution: both of these services degrade or compress the images. The images after transmitted or shared through those services are not the full-resolution image files, so they are not suitable for permanent or archival storage.
Further explanation in these links:
https://talk.macpowerusers.com/t/having-to-move-back-to-dropbox/5826/22?u=arthur
https://talk.macpowerusers.com/t/shared-photo-albums/18935/5?u=arthur