To resize an image, click on the marked corner of that image and drag that corner out to make the image larger. Drag the corner in to make the image smaller. Release when you like what you see :) Don’t forget to Save your changes!
Note: Banner images are always the full width of your email (590 pixels). So, those can’t be resized. All other images can range from 79 to 530 pixels in width (height is not limited).