/* ----- GALLERY HACK ------- */
remove_shortcode('gallery');
add_shortcode('gallery', 'dz42_medium_gallery');

function dz42_medium_gallery($atts) {     	  
	$atts['size'] = 'galerija-thumbnail';
	return gallery_shortcode($atts);
}

change wordpress default gallery thumbnail size

Bravo Tomo! :)