Hi Clements,
If these are featured images, then you can change their source here: https://i.imgur.com/RITc2il.png
If the images are mostly fetched from the text content, or custom fields etc.., then there is a built-in image caching, that you can turn on here: https://i.imgur.com/aRFL9gP.png
I only recommend the image cache, if the images are not the featured ones, otherwise changing the image size is much faster.
