You can add missing variants from the original comic's page. There you will find a link to "Submit New Variant Cover" under the Contributions dropdown.
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This submission form allows you to submit the variant cover, set a title, price, UPC, and more. Fill out the form as well as you can and it will be submitted to our moderators for review. You will be able to monitor its approval state by referring to the My Contributions area of your profile. When it is approved, it will begin appearing in the database and to other members of the site.
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Please note: If you would like to update/fix an existing cover image, you will need to submit it as a correction to that cover's page.