If you use iCloud Photo Library, iOS has a feature called Optimize iPhone Storage to help save storage space on your iPhone. Your original photos and videos are kept in iCloud Photo Library, and optimized versions are kept on your iPhone.
Macworld reader Alexandra asks about optimized photos, iCloud Photo Library, and uploads:
I have Optimize iPhone Storage turned on on my iPhone. If I decide to order some photos from my iPhone through an online photo processor, will the photos be uploaded to the web site in their full or reduced resolution?
This is a terrific question, because the answer is completely non-obvious in the interface. Whenever you use a photo selector in any app, or browse in Photos, the originals of any images or videos that you select get retrieved from iCloud before they’re available for upload or manipulation.