Within the Drupal Metatag module, you frequently want to assign images to the og:image or twitter:image metatags. However, you don't want to serve the original image.
You can chain the image style after the image token to serve an image style.
Replace field_image with your image field machine name, and replace my_custom_image_style with the machine name of your image style. This works for image fields and media fields.
[node:field_image:my_custom_image_style:url]
Alt text:
[node:field_image:alt]