under the galleries folder is a subfolder for each different gallery.
in each of those folders is an index.php page.
in the index.php file, there will be a line similar to:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=132c&full=782p'));
that initializes the gallery and tells it what size the images should be.
tells it to use the 132 px wide cropped image for the thumbnail and the 782 px wide proportional image for the large image.
the available choices are: