iCloud Photos and iCloud Backup are two different things.
If you have iCloud Photos turned on, your photos will sync to iCloud whether or not you ever do an iCloud Backup. Pausing an iCloud Photos sync doesn't affect iCloud Backup.
If you have iCloud Photos turned on, your photos are not also included in an iCloud Backup.
You can check the next backup size as described below from If you can't back up to iCloud - Apple Support
Check your iCloud storage and Next Backup Size in iOS or iPadOS
- Go to Settings > [your name].
- Tap iCloud. The amount of iCloud storage that you've used is listed at the top. Learn more.
- Tap Manage Storage.
- Tap Backups, then tap the device you're using. Next Backup Size is listed under Last Backup and Backup Size.
It may take a few minutes for your device to calculate Next Backup Size.
Also be aware that iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See: https://support.apple.com/en-us/HT204264
Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.
Another way that photos may be on iCloud (if you don’t have iCloud Photos turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. The contents of backups are not viewable at iCloud.com and if you delete a photo from a device it will not be included in future backups and will be lost.
