Are you using the same Apple ID on both of your device? If yes, just back your iPad picture on iCloud. Then open your iPhone go to picture app, and tap "shared".
1. Make sure both devices are configured with the same Apple/iCloud ID
2. Make sure either Photostream or iCloud Photo Library is enabled on both devices.
3. Make sure that both devices are connected to the Internet.
4. Give the devices time to sync.
5. Understand that sometimes Apple experience problems with their servers.