Add this code into your them’s funtions.php file.
if ( function_exists( 'add_theme_support' ) ) {
add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 150, 150 ); // default Post Thumbnail dimensions
}
if ( function_exists( 'add_image_size' ) ) {
add_image_size( 'category-thumb', 300, 9999 ); //300 pixels wide (and unlimited height)
add_image_size( 'homepage-thumb', 220, 180, true ); //(cropped)
}
Then just change “category-thumb” or “homepage-thumb” to whatever you want, or leave them as is and change their dimensions. Then you replace “thumbnail” with what you named the new thumbnail size. So using the code above, your shortcode would look like:
[display-posts tag=”advanced” image_size=”category-thumb”]
instead of the default:
[display-posts tag=”advanced” image_size=”thumbnail”]
I cant remember, but you may need to set a featured image for this to work as well.