When working with a WordPress theme, it often looks good to have images in a page be the same size. The default image sizes do not create an exact cropped image, but resize based on the original proportions of the image (except for thumbnails). You can add a custom image size and make the cropped parameter true. As long as the image you upload is bigger than the image size, all your images will be resized and cropped if necessary to your image dimensions.
Adding a custom image size is easy in WordPress, but these image sizes do not appear in the dropdown when you are adding images to a post or page.
Here is a snippet of code for your functions.php file that adds a custom image, cropped to a specific size, and then adds the new image to the list of standard WordPress sizes.