Where is the back up photos of icloud

If you have been syncing photos with iCloud, they are at iCloud.com/Photos. Another way to download them to your computer if you are using OS 10.10.3 or above, is to open Photos, then go to Photos/Preferences/iCloud and set up syncing. Then go to System Preferences/iCloud, sign in, and check Photos syncing.

If you weren't syncing photos with iCloud, they are part of your backup. If you need to recover them from the backup, you will need to restore the backup.

Restoring from a backup erases all data currently on your device. Any data newer than when the backup was made will be erased. A restore is an all or nothing restore.

Backup - Restore your device from an iCloud or iTunes backup

How have you been backing them up to iCloud ? If you are using iCloud Photos and want to download them on your computer : Archive or make copies of the information you store in iCloud - Apple Support