How do I control uploads and downloads?

I have a data use problem because of a data cap on my ISP. How can I tell my devices when to upload and when to download photos with iCloud and, regarding downloads which photos? The only control I'm aware of is turning iPhoto off in the cloud preferences. When I turn it back on, hoping to update with the photos since it was last on, I get a message that 7000+ photos are downloading. OTOH, if I leave it on all the time the 50 photos taken to get a good one are all uploaded and then again downloaded to all devices. That busts my data cap real quick.

I want to tell my phones when to upload and and my iMac to download the full picture on my computer only when I ask for it. Can that be done? Should I switch to dropbox? another photo system?

A related question; when I'm browsing the photos in iCloud how much data is used to view one picture? I assume it's not the full 2-4MB that is stored in iCloud, but how much?