make sure that the update record behavior applied to the page is set to trigger on the Forms submit button.
add another button next to the image form element, name it deleteImage, set the value to return the file name value from the recordset
Add a file manipulation - > Delete file behavior.
set the trigger to use the Button deleteImage pressed
set the path to use the same path as the file manager, set the file name to use the deleteImage button value.
add another update record behavior, this one will use the deleteImage trigger again, set the value for image column to a blank space, so that it will be the only column updated.