I think you're getting icloud storage mixed up with device storage.
If you have a lot of photos on the device, that has nothing to do with storage full messages regarding icloud. Also, photo stream is not part of the icloud storage limit.
My guess is it's your device that is running low of storage, not icloud. But...
Settings>General>Usage. This tells how much storage is used on both iCloud and on the device.
Regarding photos - once they are synced to a computer via photo stream, move them out of the computer's photo stream folder and into some other album for storage. Once done, you can delete photos from the camera roll on the device.