Simply set the theme width to Variable (0px horizontal margin) and you will find all width constraints are removed. An image will stretch to the full width of the screen.
Should be the same in most of my themes, including Flood and Volcano.
If using Stacks to place an image, don’t forget that by default Stacks 3 will set a maximum width and height of 1200px for images dropped in. To remove this, simply put the image into edit mode and uncheck the constrain settings.
If you need more flexibility for the positioning of fullscreen images (like if you want to have an image cover the entire background with no whitespace showing) then try addons like the RWExtras Background Stretcher or Full Screen Slider stacks and the ThemeFlood Photogenic or Splash themes. All are ideal for the creation of simple splash / landing pages.