If you were using iCloud Photos they are saved in iCloud, not in a backup.
Set up iCloud on your iPhone, iPad, or iPod touch - https://support.apple.com/HT204136
What does iCloud back up? - https://support.apple.com/HT207428
About backups for iOS devices - https://support.apple.com/HT204136 - including a list of what is backed up.
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iCloud backups include nearly all data and settings stored on your device. iCloud backups don't include:
Data that's already stored in iCloud, like Contacts, Calendars, Notes, iCloud Photos, iMessages, Voice Memos, text (SMS) and multimedia (MMS) messages, and Health data*
Data stored in other cloud services, like Gmail and Exchange mail
Apple Mail data
Apple Pay information and settings
Face ID or Touch ID settings
iCloud Music Library and App Store content (If it's still available in the iTunes, App, or Apple Books store, you can tap to re-download your already purchased content.)