To do this you can log into the mysql database and use the following commands:
use jamfsoftware;
delete from attachments where attachment_id=#;
delete from attachments where attachment_id=#;
Where # is the id of the attachment you would like to delete. To obtain this you can access the JSS web application and navigate to your icons, right click on the icon and at that point you can “Copy Image URL”, this url will contain the id we need to use to delete.