gallery section cropping thumbnails

On the front page gallery section, using the grid display, my images are getting cropped. Some square images have been cropped into rectangles with an 8:5 ratio.
I looked in my uploads folder and some of the images have not only the original/ large/ medium/ thumbnail, there are other image sizes in there as well with 8:5 ratios. I gather that this is function of the theme.

Note: the gallery images are not cropped on the page where the gallery is located.

But how do I get the gallery section on the front page to also display uncropped images?

(I’m guessing I need to edit the php to call up wordpress’s medium sized image and not onepress’s medium-sized cropped image?)

Thanks