Image size issue — particularly on mobile

I'm getting messages like this in Page Speed Insights: This image file is larger than it needs to be (1024x819) for its displayed dimensions (291x231). Use responsive images to reduce the image download size. I thought Catch Responsive Pro handled that.  What's going on here? It also tells me: "Using a modern image format (WebP, AVIF) or increasing the image compression could improve this image's download size." I have WebP images available. How do I get Catch Responsive to use them? Thanks. Cindy