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<?php
// COpy after
/** 
 * Snippet Name: Disable auto creating of image sizes 
 * Snippet URL: http://www.wpcustoms.net/snippets/disable-auto-creating-image-sizes/ 
 */  
 function wpc_unset_imagesizes($sizes){  
    return array( $sizes['thumbnail'], $sizes['medium'] );
}  
add_filter('intermediate_image_sizes_advanced', 'wpc_unset_imagesizes' );