Dropdown value with an image

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1

I have a dropdown populating from a custom post type. I would like to pull the featured image as a value with the post titles. I thought i would be as easy as changing

$choices[] = array( ‘text’ => $post->post_title, ‘value’ => $post->post_title );

to

$choices[] = array( ‘text’ => $post->post_title, ‘value’ => $post->wp_get_attachment_url( get_post_thumbnail_id() ) );

Thanks