The thumbnail only shows up for the linked records field when the view is set to either Gallery, List, or Kanban. There are two options on where to get the thumbnail for the linked records field:
Attachment from Form Fields - You can have multiple attachment fields in your child form but the thumbnail can be pulled out from the first attachment field in the Form Fields.
Custom Thumbnail Field - You can use this setting to choose an attachment field that is not listed after the first attachment field in the Form Fields or even an attachment field that is not in the Form Fields.
If the linked form includes an attachment field, the first image will be shown by default.
In addition to using the attachment field, you also have the option to use a URL field as the thumbnail source. This can help you save on storage space within Airtable. However, it's important to note that the image must have public access in order for the portal to display it as a thumbnail.