After the image has loaded, resizing the window will not result in a different image being loaded. The one chosen at load-time is the one you are stuck with. It will be scaled up or down as the window is resized.
(Update: That used to be true in all browsers. As of Oct 2021, it is true in Safari but Firefox and Chrome will replace the image as the window is resized.)
Available sizes are 90×50, 180×100, 360×200, 640×360, 960×540, 1024×576, 1280×720, 1920×1080, 2560×1440 and "print".
|Image:||(current rendered size of image in points)|
|(rendered size of image at load-time in points)|
|Viewport:||(size of viewport in points)|
|(size of viewport in real hardware pixels)|