Restoring deleted photos from iPhone backup

If you create a new backup, it will replace the current backup. Restoring your current backup will replace the contents of your device. If you wish to preserve your new photos, you'll need to offload those first. Then, restore your old backup.

Neither iCloud backup nor local computer backups don't allow you to choose bits and pieces of the backup-- it's all compressed and mixed up, so you must restore all or nothing. Two backups cannot be combined.

As muguy was saying, you need to copy your current pictures to some other place, like an external drive or even to iCloud Drive. Once you are sure they are safe, then you can restore this phone (or a new one) to that older backup, and then copy those offloaded pictures back to Photos on the iPhone.

Until then, make sure that your backup isn't overwritten by a new one!

As a thought, probably it would have been better to connect Photos to iCloud, and then those older pictures would be independent of backups. That won't help now, though.