Thumbnail link to different image in gallery

I’m looking for a solution. Two images are involved here. The first is an image of a shirt from the front. The second is a larger image of different angles of the shirt design so you get an idea what it looks like.

I need the gallery thumbnail when clicked to load the 2nd larger image in the lightbox and not the larger version of itself.

I can accomplish this using the WP Gallery Custom Links plugin which allows me to put the URL of a different image there. I need to use the standard WP Gallery to do this.

Is there a modification that would allow me to do this using the theme’s gallery?
I’d be fine using the WP gallery, but I cannot find how to increase the size of the thumbnail. I changed the thumbnail size in Settings/Media and regenerated the thumbnails and that doesn’t work.

Either direction will work for me. I’d prefer the theme gallery if you have a solution.

Thanks!