Q: I want to know if I delete photos off of my iPhone 6s if they will stay in iCloud?

If iCloud Photo Library is turned on, They will still be on iCloud, but back up your phone just incase.

If you have iCloud Photo Library (Settings > iCloud > Photos > iCloud Photo Library) turned on and you delete a photo from your phone it will also be deleted from iCloud and from any other device connected to that iCloud Photo Library. You can reduce the storage space required on the phone by turning on "Optimize iPhone Storage" on that same settings screen. Apple does not currently offer any cloud-only storage option for photos. More information on iCloud Photo Library: iCloud Photo Library - Apple Support

Michael McCranie wrote:

 

If iCloud Photo Library is turned on, They will still be on iCloud, but back up your phone just incase.

That is not correct -- if iCloud Photo Library is on a deletion on the phone will also result in the photo being deleted on iCloud -- and a backup won't help as neither iTunes nor iCloud backups will include photos on the phone that are stored in the cloud. The backup information is documented in About backups for iOS devices - Apple Support and the iCloud Photo Library information is documented in Get help with your iCloud Photo Library - Apple Support