Solution 1:
If you use Photo Stream as you mentioned your photos will be stored in iCloud separately and not in a backup, therefore it is safe to delete previous backups.
To further reduce backup size iPhone backup takes too much space
Does photo stream retain deleted iCloud photos?
Solution 1:
If you use Photo Stream as you mentioned your photos will be stored in iCloud separately and not in a backup, therefore it is safe to delete previous backups.
To further reduce backup size iPhone backup takes too much space