ThrillingTheme Pics Blurry

Hello,
I am trying to set up a new website and Im using the Trilling Theme (https://www.thrillingheroics.com/thrillingtheme/). Unfortunately I can’t get the main picture that shows up above every post to load correctly. On the Thrilling Theme website it says that the pics are automatically resized. I even tried resizing them myself before uploading but they still show up blurry. There don’t seem to be any options I can change. Please help!! (website is https://www.thebucketlistadventures.com)