Reader Steven Alford asks:
When I click on a single photo on, say, my iPad, a small clock face rotates in the lower righthand corner and the photo is displayed. Is that photo downloaded to my device and does it take up room on the device? Or, are only photos taken by the device stored on the device?
If you’re using iCloud Photo Library, you can set synchronization such that Photos in iOS and OS X only caches thumbnail previews of images. (In iOS: Settings > Photos & Camera and set Optimize iPhone/iPad Storage. In Photos for OS X: Preferences > iCloud, and set Optimize Mac Storage.)
Each OS decides when to dump full-resolution versions of photos, even those imported, uploaded, or taken by the device on which Photos is running, after those images are uploaded to iCloud. This can happen immediately if storage space is low.