The images size is configurable in the theme’s options (either directly or dependent on other theme options). You should ensure that the images you use are of that size.
If you use columns from posts, the theme will auto-generate crops from the posts’ featured images, so make sure the featured images are larger than the column images.
If you’ve uploaded files to the library before changing image size values, you’ll need to regenerate already created crops to ensure the theme has images of the correct (new) sizes.