How change thumbnail size displayed in default Gallery

I like the clean look of the default gallery. I uploaded pictures at the size that I would like to display when the “special” page is displayed on clicking and that option is selected.

However, the images that show-up in the Gallery are all square (150 X 150). I would like them to be proportional to the original image as too much info is cropped off. Somehow, I have some images in the media panel that show a different (proportional) thumbnail whenever they are displayed and the info window shows that the thumbnail is , indeed, not the standard square default thumbnail. So , I guess it can be done but I have no clue how that happened. =)

Anyway, my need is to show the images that display in the gallery to be small versions of the larger images not squares.

Thanks in advance for all help