The simplest way to do this is to edit the existing graphic. You can download the image by right clicking it and saving to your computer. Then edit that image to look however you’d like it to. This will keep the image dimensions the same as the theme default. Use the Image Manager under the Pages tab to upload your edited image into your images folder. Your uploaded version will overwrite the default image. If you need to edit the overall style of your site, you will have to edit the styles.css page.