How to store photos in iCloud?
Prior to a recent change, Apple's solution for this would be to ensure that iPhoto was running and you have configured iCloud to automatically import all photos into your library.



Now that there is no limit to the number of photos, you just need to remain under the current upload limits to avoid having photos that are only on your device and not backed up to Photo Stream:


25,000 photos per month
10,000 photos per day
1,000 photos per hour


Other than temporary pauses, you should be able to store as many photos in the cloud as your have bandwidth and time to upload.