Scaled Images

I received this error from GTMatrix:

“The following images are resized in HTML or CSS. Serving scaled images could save 92.5KiB (86% reduction).

https://sisterhoodagenda.com/wp-content/uploads/2018/11/A-Journey-Toward-Womanhood-updated-cover-2016-782×1024.jpg is resized in HTML or CSS from 782×1024 to 286×375. Serving a scaled image could save 92.5KiB (86% reduction).”

This image is in our footer-is that why it received an “F” rating? How can I “scale” this image or reduce it as suggested?