Wordpress creates a new image URL for each of the sizes of images it creates when an image is uploaded. It adds the image size in pixels to the end of the string before the image extension. The problem is that if say an image size for a medium image is 300x300, wordpress may crop it to say 300x180 to keep it in dimension.
This makes it difficult when programatically fetching a larger version of an image thumbnail as the file ending -300x300.jpg may not exist.
Is there a way of fetching the image by using the URL for the full image, with a PHP variable eg. ?size=medium on the end that will load the correct version. I'm sure I have done this with older versions of wordpress but cannot find any documentation to prove this.
From the documentation:
<?php echo get_the_post_thumbnail( $post_id, $size, $attr ); ?>
$size
Either a string keyword (thumbnail, medium, large or full) or a 2-item array representing width and height in pixels, e.g. array(32,32)
https://codex.wordpress.org/Function_Reference/get_the_post_thumbnail
Add this to functions PHP so that the line above works for the first image if the post does not have a featured image.
function set_first_as_featured($attachment_ID){
$post_ID = get_post($attachment_ID)->post_parent;
if(!has_post_thumbnail($post_ID)){
set_post_thumbnail($post_ID, $attachment_ID);
}
}
add_action('add_attachment', 'set_first_as_featured');
add_action('edit_attachment', 'set_first_as_featured');
You can register a image size to use anywhere in your theme with add_image_size, see:
add_image_size( "your-custom-image-size", 300, 300, true );
then you can use in your theme as well:
the_post_thumbnail("your-custom-image-size", $attr);