Display full image in archive excerpt

Hello,

I am working on a page (using the Nisarg theme) that is not yet live. I have posts that are simply a photo, a caption, and possibly a short description. I want to have an archive page that will display all posts with full-sized images with the post content. Currently, it seems the theme will only display a featured image on an archive page, and the featured is cropped. Using a featured image also duplicates the image on the post page, which I do not need.

I’ve done this years ago with a different theme, but I’m not sure how to get this to work with Nisarg. Could someone please point me toward a solution? Many thanks!