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Firstly, I'm not a developer, mostly a website designer and CSS hacker.

I'll try to explain better:

I'm using a grid theme that creates a thumbnail out of the featured image to display as a post teaser. My previous theme would do the same, but would use the custom thumbnail that I created in the Thumbnail Setttings in the Media Library.

I imagine this is a simple change, maybe in the functions.php file?

I looked in the existing posts, but could not find a suitable answer. Feel free to link me to an existing thread discussing this.

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Do you mean you want to display the Thumbnail size that you set in Settings -> Media Settings? – Ahmad M Feb 5 '13 at 21:27
Can you locate the piece of code responsible for "creating a thumbnail out of the featured image" ? This is possible using Firebug and searching inside the theme files. – brasofilo Feb 5 '13 at 21:28
@Ahmad M, In essence yes, I would set the thumbnail size to match my theme's grid pattern, then ensure any thumbnails I crop are either made to that size, or at least at that ratio. – Matt Lorenzi Feb 6 '13 at 0:13

If your theme is using the_post_thumbnail() to display the featured image, adding the following code in functions.php file will display the Thumbnail size of the image (as set on Settings -> Media Settings):

add_filter('post_thumbnail_size', 'my_thumbnail_size');
function my_thumbnail_size() {
    return 'thumbnail';

Or you can display the featured image in any width (X) or height (Y) you wish:

add_filter('post_thumbnail_size', 'my_thumbnail_size');
function my_thumbnail_size() {
    return array(X,Y);
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