I did a backup of my iPhone and would like to download my photos to my iPad from the iCloud backup.how do I do this?
You can't selectively choose files to download from a backup. You can only do a full restore from a backup to a device. That means a full wipe of the device and a restore from a previous backup. Is that what you are asking to do?
How do I share my photos to iCloud. Old photos
You can only add historic photos to photo stream using a computer, it is not possible from an iOS device.
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