Add custom image sizes to drop-down selector

function.php

// Tell the media panel to add the new size to the dropbown
function custom_image_sizes($sizes) {
$addsizes = array(
'logo' => __('logo'),
'grid_12' => __('slides')
);
$newsizes = array_merge($sizes, $addsizes);
return $newsizes;
}
add_filter('image_size_names_choose', 'custom_image_sizes');