Featured images can be used for both posts and pages. The images should be at least 800 pixel wide and 400 pixel high.
On pages and single posts the featured images will be displayed covering the whole width of the window in the header. The maximum height of the image is 400 pixels. The original uploaded file is used for the featured image.
On blog, archive and search pages the featured image is displayed above the title. They are shown in proportion 2×1. These images are cropped to the size 800 x 400 pixels on uploading.
If there have been images uploaded before the theme was activated then the theme will need additional image sizes. You can generate the additional image sizes by running a plugin like Regenerate Thumbnails.