automatic image resizing *only* on archives.php and mainindextemplate.php

Hi, here’s what I want to do..

My site: https://www.audioscribbler.co.uk

In posts, artwork images are 240×240, which is a good size for in the post itself.

When a visitor clicks on ‘reviews’ to browse the old reviews, images are still 240×240. I want these to be small – thumbnail size.

I achieved this before with viva zoom but it was messy and I had the code on another PC. Anyone got any suggestions? thanks