Se ho capito cosa devi estrarre, prova con:

codice:
function str_image($img){
    preg_match('#img_id=(\d+)&7=(\d+)#', $img, $match);
    return array(
        'img_id' => $match[1],
        '7'      => $match[2],
    );
}

print_r(str_image("<img src=\"/image/image_gallery?img_id=3130048&7=1275387149570\">"));