Doesn't matter what I set in the_post_thumbnail() function it shows only thumbnail. I dig around and found ngg_post_thumbnail() in post-thumbnail.php which is responsible for outputting image HTML.
The function doesn't check
the_post_thumbnail('thumbnail'); // Thumbnail (default 150px x 150px max)
the_post_thumbnail('medium'); // Medium resolution (default 300px x 300px max)
the_post_thumbnail('large'); // Large resolution (default 640px x 640px max)
the_post_thumbnail('full'); // Full resolution (original size uploaded)
borrowed from http://codex.wordpress.org/Function_Reference/the_post_thumbnail
Since I the options in NGG panel only has thumbnail, Full, and SinglePic, I just replace $img_src = $image->thumbURL; around line 121 in post-thumbnail.php with the following code.
if($size === 'full' or $size === 'singlepic')
{
$img_src = $image->imageURL;
}
else
{
$img_src = $image->thumbURL;
}
After this you can use
the_post_thumbnail('full');
or
the_post_thumbnail('singlepic');
Hope this will help some people.