First figure out what program your using to auto backup, if it's your windows operating system then look in the control pannel under backups and find out where you are telling it to store your backups you can then manually go to that location and delete the backup files which are usually compressed files so you would likely need to delete the whole thing. You could possibly change the backup settings to tell it not to include your pictures folder if this is what you want to exclude from the backup then run a new backup.
Answers & Comments
First figure out what program your using to auto backup, if it's your windows operating system then look in the control pannel under backups and find out where you are telling it to store your backups you can then manually go to that location and delete the backup files which are usually compressed files so you would likely need to delete the whole thing. You could possibly change the backup settings to tell it not to include your pictures folder if this is what you want to exclude from the backup then run a new backup.