In Views trying to change marker styles..

I have a field: ${field_allowed} which returns: "Y"
But I need it to return "Y.png" so externalGraphic will return as the marker style.
So I re-write the result.
Now ${field_allowed_rentered} returns: "Y.png" and ${field_allowed} returns "Y".
But externalGraphic still only returns "Y".

How can I set externalGraphic to return the field_allowed_rendered version? All the style setting allows me to select is field_allowed.

Also- if a select list field has no value - what is returned? "-none-" ?