Inject Feature Image

Heh. As always – chances of solving the problem on your own increase with desperation and amount of posts published about it on the net. Finally I’ve solved this issue. Here is the code added to the functions.php of the child theme.

function _jdutils_inject_thumbnail( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
    if ( empty( $html ) ) {
		$post = get_post($post_id);
		$html = "<img src='someimage.jpg' atl='$post->post_title'>";
    }
    return $html;
}
add_filter( 'post_thumbnail_html', '_jdutils_inject_thumbnail');

Still I’d love if someone more competent than me could look at it and confirm this is the best way to go about it.
Thank you.

The code works, but it’s a very “naive” solution. There are two main problems that I have with them:
1. I have some conditions which define if the injection should happen or not. When I do not want to inject anything I’d like it to behave like it normally does without the add_filter (so what your theme provides for posts with no feature image). Unfortunately when I simply return empty $html from my function it causes a blank result, which is different from theme’s default (icon representing type of post. I’d like this to be as if there was no add_filter when I don’t want to inject anything. I tried remove_filter, but since that happens from withing the function after checking is done it changes nothing.
2. When I inject $html with the tag than no matter what I add as alt or title in the , the template always takes part of the content (usually excerpt) as the alt/title parameter. I think it has something to do with the <span> tag that the is wrapped in which to my understanding is causing the hover effect.

And the hover effect leads me to another, less urgent problem, but still important – how do I turn it off and get rid of the link (which points to the image file itself in a lightbox) when viewing the post (is)singular / is_single / is_page)?