thumbnails are of different sizes/formats

I have just yesterday installed Minimatica as a new theme on my photography website (http://photography.alicetragedy.org)
I’ve had this blog/portfolio for a couple of years, so there already quite a few images uploaded and posts written.
My question is this: whereas on the posts with galleries created prior to the theme change the thumbnails are rectangular, everything uploaded AFTER the theme change will automatically create a SQUARE thumbnail. I have nothing against that, since I find square thumbnails more clean anyway, however in some galleries the result is like this:

http://photography.alicetragedy.org/?p=1222

you can see that the thumbnails at the top are rectangular, and the ones at the bottom (added just a couple of hours ago) are square. doesn’t look very nice…
is there any way to change this in the options of the theme or something? it doesn’t look like it’s anything to do with the media gallery itself, I really feel like the theme is “forcing” some default, without however changing the old posts/thumbnails.

any help or tip would be appreciated, thank you so much, and thank you daniel for a lovely, gorgeous, and clean theme.