DropboxA great storage provide which I use already a long time. Recently they started focusing more on the storage of photos for example automatic storing of photos from your mobile in your dropbox folder. If you are not a dropbox user yet, use this url to get 500Mb extra (also for me so thanks!).
FlickrOne of the best known photo storage providers so it seemed obvious to include this one first.
I included these new sources in the premium app. If you are a user of the ad-supported app, you will have to install another app which includes these new sources but not network drives or UPnP. Both apps do include the possibility to do a slideshow of local photos.
Let me know what you think, what is missing or what other providers I should include!