Search criteria for empty data field
I have a large online store where data is being input in various stages by different people, one may upload the item details and another the picture etc. I'm creating a status report for items and want to be able to search for items that do not yet have a picture so that it can then be updated. At the moment I can search for items that have a picture by using Includes '.jpg' but how can I search for items where the datafield is empty?