Scaled featured images in home instead of resized

I checked wwww.gazeboinferro.com on gtmetrix and it say that the biggest problem to load it faster is that the images are resized trough html and not really scaled. how to patch it in your theme?

The following images are resized in HTML or CSS. Serving scaled images could save 219.2KiB (62% reduction).

http://www.gazeboinferro.com/wp-content/uploads/cesena.jpg is resized in HTML or CSS from 714×374 to 458×240. Serving a scaled image could save 79.8KiB (58% reduction).
http://www.gazeboinferro.com/wp-content/uploads/copertura-policarbonato-9.jpg is resized in HTML or CSS from 606×454 to 320×240. Serving a scaled image could save 61.6KiB (72% reduction).
http://www.gazeboinferro.com/wp-content/uploads/Custom-Gazebo_thumb1.jpg is resized in HTML or CSS from 509×383 to 318×240. Serving a scaled image could save 52.4KiB (60% reduction).
http://www.gazeboinferro.com/wp-content/uploads/copertura-policarbonato-3.jpg is resized in HTML or CSS from 576×365 to 378×240. Serving a scaled image could save 25.3KiB (56% reduction).
http://www.gazeboinferro.com/wp-content/uploads/cropped-Small_white_square1.png is resized in HTML or CSS from 600×300 to 240×120. Serving a scaled image could save 136B (84% reduction).