By default, when you delete some images from admin panel or front end, these images are deleted softly, that is to say, they are only marked as deleted status and will be actually deleted from physical server only you run the cronjob script:
And another IMPORTANT issue is that cronjob.php only deletes 200 images tagged as "deleted" per running! This was designed for saving server load and mysql queries! So you may run cronjob.php every often to delete lots of images. DO NOT expect to run it once to get all images deleted! It is better to setup a crontab to execute the script periodically!