SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/for-developers-delete-images

DeleteSingleImageNativeAPI.cs

using System;
using System.Linq;
using Telerik.Sitefinity.Libraries.Model;
using Telerik.Sitefinity.Modules.Libraries;

namespace SitefinityWebApp
{
    public class DeleteSingleImageNativeAPI
    {
        private void DeleteImageNativeAPI(Guid masterImageId)
        {
            LibrariesManager manager = LibrariesManager.GetManager();

            //Get the master version of the item.
            Image master = manager.GetImages().Where(i => i.Id == masterImageId).SingleOrDefault();

            if (master != null)
            {
                //Mark the item to be deleted.
                manager.DeleteImage(master);

                //Save the changes.
                manager.SaveChanges();
            }
        }
    }
}