How do I retrieve photos saved to iCloud?

iCloud Photo Library is a syncing service where the photos are stored both on your device and on iCloud.com; on an iPhone, iPad, Mac you can view the pictures in the Photos app on the device. You can also use any computer and log in to www.icloud.com and go to the Photos app there. See: iCloud Photo Library - Apple Support

If you didn't have iCloud Photo Library turned on but did have iCloud backup turned on in an iOS device, you can restore the entire phone (not just the photos) from iCloud. Note that the restore will replace all of your data on the device with the data from the time of the backup. iCloud: Restore or set up your iOS device from iCloud