Featured images not full size

Hi, I've been using Jomson Pro for a while. I cannot seem to get the featured images on the main page to show full size. The images I'm using are 350px x 350px but they show up much smaller on the site. Is there something I'm doing wrong? This is my site so you can see: https://www.WhiteRosesArt.com Also, Google page speed insists those 4 featured images aren't optimized but they are. I did it manually in Photoshop and I have a plugin that auto-optimizes my pictures when I upload. Do you know what is causing this? Google doesn't take exception to any of the other graphics on the main page, just those 4 featured images.