You need to have appropriate image size, otherwise close matched size will be used. You can put a line in your themes functions.php – add_image_size( 'custom-size', 70, 50, true ); and regenerating all of your images using this plugin – http://wordpress.org/plugins/regenerate-thumbnails/.
Ah, now I understand. [post_thumbnail] uses the thumbnail generated from the post ‘featured’ image. Hence your suggestion for regenerating the thumbnail? I originally thought that your plugin auto-resizes the first image found in the post – my bad assumption.
Thank you.
You can use bulk regenerate thumbnails just to make sure expected size (70x50px) are created and exists.
