Side by Side images

I been using this plugin for a while and it works perfectly.
When i test the site on GTMetrix, I get this:

Serve scaled images

The following images are resized in HTML or CSS. Serving scaled images could save 183.7KiB (88% reduction).

I’ve found that resizing the image, uploading it to my site and setting the resized image as the featured image solves this problem. But the original image the plugin had been pulling and the resized image i set as the featured image are being displayed side by side in the post excerpt. How can i fix this?

https://www.ads-software.com/plugins/thumbnail-for-excerpts/