After installing your theme, you should regenerate the thumbnails (featured images) on your site. This will make sure your images are sized/cropped to match the sizes defined in your theme. To do this, use the Regenerate Thumbnails plugin.
After you’ve installed and activated the Regenerate Thumbnails plugin, go to Tools > Regenerate Thumbnails.
Then click the Regenerate Thumbnails button to begin regenerating the featured images on your site.