You can actually leave the size at 1140px by 400px instead of 1138px if you want (it will be perfectly centered if you do 1140px). Once you add an image, and change the size in your settings, you need to use a plugin called Regenerate Thumbnails – http://wordpress.org/plugins/regenerate-thumbnails/ and it will automatically resize all of your images to match the sizes you have in your settings. You only need to run this plugin if you change a size somewhere, but you wont need to do this everytime you add an image. So try and change it to 1140px and 400px if thats what you want, and then Run the plugin and it should fix your issue.
.
