The Image Size Selection plugin is designed to add all available image sizes to your Media size selection drop down window. By default, windows generates a Large, Medium and Thumbnail image size (See Settings/Media for your setting sizes). Most modern themes, however, include a wide range of additional sizes.
Most themes had the add_image to the functions.php file, which adds additional images to your site. These are often used in widgets or specially designed areas of your theme. By default, these images are not included as an option in your drop down.
