Custom Image on the Customer Record

Does anyone know how to generate an image in a custom image field on the customer record based upon the value in a list field?  Example: if custom field equals 1 (list field with selections of 1-2-3-4-5) display a picture from the images folder on the customer record in an image field.

I thought this might be relatively easy to do with a DECODE or CASE sql expression, but when an image custom field does not have a "default value" or formula fields to use SQL (available in other custom field types but not visible for Image). 

example: CASE WHEN {list value} = '1' THEN urlforpicture1 WHEN {list value} = '2' THEN urlforpicture2 WHEN {list value} = '3' THEN urlforpicture3 WHEN {list value} = '4' THEN urlforpicture4 WHEN {list value} = '5' THEN urlforpicture5 ELSE null END