Dave Martin is hitting a roadblock with Photos for OS X and iCloud Photo Library. He uses the optimized setting in iOS and OS X to reduce the storage consumed by the full resolution of photos and videos.
I would like to know how to download the full-resolution version of a photo if you have chosen to keep only thumbnails on your Mac. And, also, if you do have the full-resolution version of the photo on your Mac how to copy that version to a flash drive.
It’s not an obvious process and there’s no menu item. You also can’t search to find out whether an image (or movie) is downloaded at its full resolution. The only way to be sure? Double click the image to view it in Photos for OS X in some larger-than-thumbnail view. If it’s already cached locally at its highest resolution, nothing happens; if it’s not, you’ll see a circle in the lower right of the image begin to fill in (starting 12 o’clock and going through 360 degrees as the download proceeds).
Read more here:: How to force a full-resolution iCloud Photo Library download