You would have to recursively sum all subalbum numImages.
something like this should work:
`function totalImages($album) {
global $_zp_gallery;
$sum = $album->getNumImages();
$subalbums = $album->getSubalbums(0);
while (count($subalbums) > 0) {
$albumname = array_pop($subalbums);
$album = new Album($_zp_gallery, $albumname);
$sum = $sum + totalImages($album);
}
return $sum;
}`
Caveat: I have not tried this.