is iCloud like a Dropbox where you can back up photos and then delete from the device to free up space, then add more or delete from iCloud as you please?
if I back up my device and delete the files off it when I back up again will the deleted files from the device also delete from iCloud?
iPad Mini
No, iCloud Photos library does not work that way. If you delete from on any device, you will delete if from iCloud and all devices. To minimize the space the photos are taking on iOS devices, Go to Settings > Photos & Camera and select optimize iPhone (or other iDevice) Storage. This will make a small pic on the iPhone that is good for viewing on iPhone but takes less memory. The more photos, the more iPhone optimizes. This is Apple's solution to freeing space.
Can I save photos on iCloud and then delete from my device like Dropbox?
