[Theme: Hueman] default gallery using resized thumbnails

Hi,

using WP default gallery within Hueman theme, setting different columns number results in having just 150×150 thumbnails awfully resized, see:
1 column – http://i.imgur.com/IMWaSoW.jpg
2 columns – http://i.imgur.com/ek7O5VG.jpg

Is there a way to have good quality gallery thumbnails on 1, 2, 3 columns and not having the gallery to use the standard one 150×150 resized?

Thanks
Patrizio