I've never used photos on the desktop or I've never syned my photos on icloud. How exactly does this work? I've basically backed up my photos on an external drive every time I get a new Phone. I want to go through all my old pics and basically chose my favourties (Probably thousands of them) and sync them across all my icloud. So should I drag all my pics from my external drive to the photos app and then organize ? Will they sync to my phone? Why do all the pics get stored on one large file under photos? Should I back this one file up after I decide what pics I want to keep?