Thank you for allowing me to assist you, my name is Joe and I will be happy to assist you with your concern today.
You can simply delete those pictures and then backup the device again. For the previous backup confirm the new backup is saved and you can then delete that old backup, the new backup will still be there without the pictures that you deleted before the backup. Otherwise you would need to go into the backup and attempt to locate those files to delete which is much easier down the way I explained.
Thank you!- Joe
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