Below the Sizes field you can choose multiple image styles so the browser can choose the best image file size to fill the space defined in the Sizes field. Typically you will want to use image styles that resize your image to have options that range from the smallest px width possible for the space the image will appear in to the largest px width possible, with a variety of widths in between. You may want to provide image styles with widths that are 1.5x to 2x the space available in the layout to account for high resolution screens. Image styles can be defined on the <a href=":image_styles">Image styles page</a> that is provided by the <a href=":image_help">Image module</a>.