Cei de la Wordpress au definit o constanta "ABSPATH" calea directorului de genul: /home/<user>/public_html/

Deci ai putea incearca:
Cod:
$strFilePath = ABSPATH . "wp-content/themes/dailypress/images/pheader/".get_the_ID().".jpg";
<? if (file_exists( $strFilePath )){?>
<img src="<?=$strFilePath?>" alt="<?=the_title();?>" />
Sa-mi zici daca nu a mers... sa incercam altfel.