Deleting Old IOS File Backups

Items that are in a backup won't be deleted - you should consider the following:

1. if everything in that backup is now in iCloud, you can delete it
2. if you delete stuff in iCloud, you MAY end up deleting it on all your devices, but that isn't the case here
3. what is in your photo library on the Mac is not affected by deleting an iOS backup
4. if there is anything in that backup that is NOT in iCloud, you won't have a backup of it if anything happens to your phone

If I delete this backup will I lose all my photos?

The simple answer to that question is no.

The other answers are all correct and valuable; they were just more comprehensive.

You won't lose content that isn't in the backup file, but should back the Mac up anyway.

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If there are copies outside the backup, yes.

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