I don't see any need for that. All you need is a sorted portal or a sorted relationship. You can define an auto-entered serial number field in this same table. It will not only serve as a much needed primary key, but you can sort either the relationship or a one row portal in descending order by this serial number field and the most recently added record becomes the first related record. If you sort the relationship, you can add the container field from the related table directly to your layout. (Note if you have a date field, you can probably sort in descending order by date field as long as you are absolutely sure that more than one record with the same Shot_ID )will never ever be imported with the same date.
I am assuming the following relationship:
Main::Shot_ID = Images::Shot_ID
I totally forgot that you can sort relationships! Thanks!