In Grand Photography theme version 2.8 or higher. We added options to customise each gallery, blog templates image size. You can open Appearance > Theme Setting > Images

You will find default image size settings (see above screenshot). You can change each image dimensions settings. These settings affect the display and dimensions of images in your gallery, blog pages – the display on the front-end will still be affected by CSS styles. After changing these settings you may need to regenerate your thumbnails