Change post thumbnail using front end post edit form

it is much easier with WordPress inbuilt function media_handle_upload

http://codex.wordpress.org/Function_Reference/media_handle_upload

// These files need to be included as dependencies when on the front end.
require_once( ABSPATH . 'wp-admin/includes/image.php' );
require_once( ABSPATH . 'wp-admin/includes/file.php' );
require_once( ABSPATH . 'wp-admin/includes/media.php' );

// Let WordPress handle the upload.
// Remember, 'my_image_upload' is the name of our file input in our form above.
$attachment_id = media_handle_upload( 'my_image_upload', $_POST['post_id'] );

if ( is_wp_error( $attachment_id ) ) {
    // There was an error uploading the image.
} else {
    // The image was uploaded successfully!
}

You will need to specify your file control name, then you can call set_post_thumbnail function OR set post meta ‘_thumbnail_id’

set_post_thumbnail( $post_id, $attachment_id );

EDIT:

For some reason, on the post edit screen the image file does not even upload.

Can you double check that you have properly set enctype=”multipart/form-data” attribute to your form tag?

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