Apology for the inconvenience. By default, the image should be selected automatically. When adding an item to the collection, the default image should appear as preselected. However, you can change it while adding the item. If the default image is not displaying as expected, please verify that it is properly set in the field configuration.
If you have followed the correct steps but are still facing the issue, please provide me with your site administrator access. Before doing so, don't forget to take a full site backup. Let me know how you'd like to proceed.