@Kyle: Indeed, in that case the browser would download additional images as you resize it, something you should avoid.
Personally I prefer to use one single image with a size somewhere in the middle (like in the examples). This way you can scale up and down depending on the required viewport size.
This has two advantages:
1. The quality loss is small when you scale up (better than only scaling up from e.g. an image of 800x600 px)
2. You can better use it as a fallback image for browsers that don’t support the background-size property
As a note on huge resolutions, how many people would resize their browser window to the full resolution? Not many, I think.