If you're running into issues with your theme or customizations cropping your fill-sized Pinterest images, you can add this code to the bottom of your functions.php file.
***Before doing this, make sure that you have access to your site's files via FTP (the editor in WordPress is not sufficient in case problems arise) and that you are comfortable with editing your functions.php file. Any minor error in the functions.php file can result in the "white screen of death," which is easily fixed via FTP but is difficult to fix otherwise (and a nuisance as your site will be unavailable until fixed).***
Registers a new 'tp-full' image size for a full-size image.
add_image_size( 'tp-full', 9999, 9999, false );