Paul,
The way to do this would be to now use DataAssist's update single record behavior found in the server behaviors tab.
After you select update single record - set the trigger to when the delete photo button is pressed.
Pick the table you want to update and then after hitting next on the first wizard menu, you will see the bindings for each column value... simply set your column which holds the photo name to nothing or NULL depending on how your database is setup.
Best regards,
Brian