add a form around the repeating table.
add the radio button to each cell, set the value for the radio button to the ID of the image. each radio button should have the same name to create the radio group.
you will need to have 2 update record behaviors on the page.
1) to set all of the records show field to 0. in the update record behavior, set the key column to the show field and set the key value to 1. then in the update section, set it to change the show field to 0.
2) this one will set the show field for the selected image. set the key column to the image ID, set the key value to use the radio group. then in the update section set the value for the show field to 1.
