The thumbnail is available when the linked-record view is set to Gallery or List. Choose the thumbnail source in the field's View settings:
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.
When a List view is used to display the linked records, use the Thumbnail location setting to place the thumbnail on the Left or Right of each record. Right is the default. This setting appears only when a Thumbnail is selected.
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.