When composing a post, click the “Set Featured Image” link in the “Featured Image” meta box. This will open your media library. Select an image or upload a new one and save it.
Typically, your images will be displayed using the default dimensions of the image, but depending on your theme, there may be additional cropping (cropping the image to the height/width of the theme's design) or display (stretching, overlaying, etc.) options. Please refer to the theme's documentation to find these settings, although they are usually located in the customizer or theme panel.
