WordPress | Image size test function

has_thumbnail_size.php

<?php
// Works with thumbnail, medium, large and full
function has_thumbnail_size($size, $post_id) {
    if ( has_post_thumbnail($post_id) ) {
        $image_id = get_post_thumbnail_id($post_id);
        $image_attributes = wp_get_attachment_image_src($image_id, 'full');
        $image_width = $image_attributes[1];
        $image_height = $image_attributes[2];
        $media_width = $size . '_size_w';
        $media_width = get_option($media_width);
        $media_height = $size . '_size_h';
        $media_height = get_option($media_height);
        if ( ($image_width > $media_width) && ($image_height > $media_height) ) {
            return true;
        } else {
            return null;
        }
    } else {
        // No thumbnail attached to post
        return null;
    }
}

Image size test

The principle behind this is; the function grabs the original full size image dimensions, then it grabs the requested image size dimensions (as set on settings>media), then it makes sure the original size is bigger than the requested size. The idea being, if the original is bigger; it has been able to generate the smaller size.

It's not fool proof, if the image sizes have changed since the original upload, I guess the principle fails.