Add image attachment to post via coding?

You do not add image attachment to post. In WordPress image attachment is also post. See post types in Codex.

  • what is usually called post on surface is technically post of post post type.

  • image attachment is a post of attachment post type.

So what you need to do is to create image attachment (using wp_insert_attachment() and couple more related functions) and set your post as parent for it.

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