If you want to specify a photo as the feature photo for a post. Just put the following information in Front Matter.

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image:           "https://kuoe0.github.io/harmono/assets/images/2016-03-26-post-with-feature-photo.jpg"
image_info:
    creator:     "Anthony DELANOIX"
    creator_url: "https://unsplash.com/photos/b5POxb2aL9o"
    license:     "CC0"
    license_url: "http://creativecommons.org/publicdomain/zero/1.0/"

Note: To make jekyll-seo-tag plugin work, only absolute URL supported!!