sanver,
I’m showing in my environment that the images are getting resized according to the image sizes that are set in the WordPress Settings (Settings->Media). The Events Calendar uses the ‘medium’ image size by default, however, it looks like your site is configured to have smaller ‘medium’ sized images. If you change your ‘Medium’ image size in WordPress and rebuild your thumbnails using a plugin like this one, then you should see the larger images displaying.
Give that a try and let me know if it does the trick. Thanks! π
-Casey-