Wordpress , display resized image in theme with PHP - wordpress

Question

I'm coding a shortcode that accept one media ID as an attribute. This shortcode should then display the image at an existing size registered with add_image_size. Problem : my image is not displayed at the correct size.
Explanations :
My media image has been uploaded in the WP library. The original file size is huge (1227x924). The media has the ID 294.
I registered an image size in functions.php :
function my_after_setup_theme(){
add_image_size('my-image-size', 210, 136, true);
}
add_action('after_setup_theme', 'my_after_setup_theme');
I insert my shortcode in one of my pages : [my_shortcode imageid="294"]
My shortcode code is :
function my_shortcode_func($atts)
{
$a = shortcode_atts(array(
'imageid' => 0,
), $atts);
if ($a['imageid'] == 0) {
// default placeholder image
$img = 'img src="http://placehold.it/210x136"/>';
} else {
// get resized (NOT WORKING !)
$img = wp_get_attachment_image($a['imageid'], 'my-image-size');
}
return $img;
}
add_shortcode('my_shortcode', 'my_shortcode_func');
I expect the original image to be resized at the correct size (ie : 210x136) in a new thumbnail file. Instead, the $img displays the original image (1227x924) , displayed at an intermediate size via the width and height attributes of the HTML <img> tag :
What am I doing wrong ?
thanks for help.

Answer 1

Did you register the thumbnail size after uploading the media? If so, you will need to use a plugin to regenerate the thumbnails.
Additionally, I don't believe you need to wrap thumbnail size declarations as you have done, I have the following;
if (function_exists('add_theme_support'))
{
add_theme_support('post-thumbnails'); // Featured image support for posts - you may not need this bit.
add_image_size('large', 700, '', true); // Large Thumbnail - This is the bit you would need
}
So it would be worth checking if your thumbnails are actually be generated to start with, as I have never done it the way you have defined in your question.