[Theme: Theron Lite] Gallery picture size inconsistent

I am running the theme theron-lite, and I am having trouble with the gallery images displaying. Some are simply a thumbnail, which cuts of some of the vertical or horizontal display of the art pieces, while others display the full piece.

For example, the piece “Pond Gardens 1970s” is a thumbnail, while the piece “Gorman 1974” shows the full image. I am looking to have the entire gallery display the full images as “Gorman 1974” is displayed.

Since the site is primarily for the purpose of art sales and exposure, I would prefer to have the full image displayed in the gallery. Thank you for any help!

https://www.thecrowsneststudio.com/artists/vida/archive-of-the-works-of-vida/vida-1970s/ This is the gallery page.