To add product thumbnails to your order emails, open the email-order-details.php file and locate the following code snippet:
'show_sku' => $sent_to_admin,
'show_image' => true,
'image_size' => array( 100, 100 ),
'plain_text' => $plain_text,
'sent_to_admin' => $sent_to_admin,
Find ‘show_image’ => and set it to true to enable product thumbnails. Set it to false to disable product thumbnails.
Set the Size of a Product Thumbnail
Find ‘image_size’ => and set the width and height of the thumbnail in the array. The dimensions are in pixels.