how to change my child theme featured image dimensions?

Question

I am developing a child theme based on the twentyten theme and now i notice the featured image in the WP backend (wp-admin) shows the wrong scaled image.

It shows the "twenty ten" size, meaning an image scaled to x198.

How do i show the featured image of my chosen size???

regards,

Answer 1

First: why do you care the size of the thumbnail displayed in the "Featured Image" meta box on the Edit Post screen?

Second: that image size isn't defined by Twenty Ten (or any other Theme). It's defined by core, and is actually simply calling the original image size, and scaling it.

Edit

Whoops; that was wrong. :) That Meta Box is apparently displaying the 'thumbnail' image size. (And I had forgotten that one of my primary beefs with Twenty Ten / Twenty Eleven is that they screw with the default thumbnail size.)

Start here:

<?php
set_post_thumbnail_size( HEADER_IMAGE_WIDTH, HEADER_IMAGE_HEIGHT, true );
?>

So, you were on the right track. :)

Next go to the define calls:

<?php
define( 'HEADER_IMAGE_WIDTH', apply_filters( 'twentyten_header_image_width', 940 ) );
define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyten_header_image_height', 198 ) );
?>

So, the easiest way to override those defined sizes is via the twentyten_header_image_width and twentyten_header_image_height filters. e.g., in your Child Theme's functions.php file:

<?php
function wpse44268_filter_twentyten_header_image_width( $width ) {
    return '150';
}
add_filter( 'twentyten_header_image_width', 'wpse44268_filter_twentyten_header_image_width' );


function wpse44268_filter_twentyten_header_image_height( $height ) {
    return '150';
}
add_filter( 'twentyten_header_image_height', 'wpse44268_filter_twentyten_header_image_height' );
?>

These will define the post thumbnail as 150x150. Change the return values to suit your needs.

Answer 2

Changing the header image size to change the thumbnails size! You can do this without touching the parent theme file - in fact, any update will likely break your custom code. This is the point of child themes…

All you need to do is use add image size in your child theme:

 if ( function_exists( 'add_image_size' ) ) { 
    add_image_size( 'my-new-thumb', 150, 150,false );

And call this into the post_thumbnail in your template.