How can I back up deleted pictures from iCloud?

Did you already check the Recently Deleted folder?

If you still can see those photos inside this folder, restore them again.

If you're missing photos or videos in the Photos app - Apple Support

After that you can transfer the photos from the photo app inside your iCloud account to your computer.

Photos that are deleted from iCloud Photos will also be deleted from all other devices that are connected to this iCloud account.

Archive or make copies of the information you store in iCloud - Apple Support