Set up a related table with a container field, one record in this table for each country. Add a text field for the Country name abbreviations. Put your flag images in different records of this related table and link it by country abbreviation field to a match field in your main table where you want to see the image. Then add the container field from this related table to your layout. When you enter or select a country abbreviation, the corresponding flag image should then appear in the container field.
Easy when you know how!