How do I retrieve photos from the cloud after I place them there?
Both iCloud Photo Library and iCloud Photo Stream are synchronisation services, and will do exactly that. Sync between devices.
Meaning that if you delete your photos from your iPad manually, they will also be deleted in the iCloud Photo Library unless you disabled this feature first.

Your best shot at retrieving your images is to check your "Recently deleted" folder on the iPad or after logging in to https://www.icloud.com under Photos.