Sometimes when theming on some level you need to output some image through imagecache, so here is how to do it. Usually in some function in template.php you will write this
$node_img = theme('imagecache', 'Nletter', $node->field_naslovna[0]['filepath'], $node->field_naslovna[0]['alt'], $node->field_naslovna[0]['title'], null);
Where "Nletter" is the name of imagecache and "field_naslovna" is the name of the CCK field.