If you are using Safari 3, please resize the viewpoint an observe the image of the Galaxy below.
The first two images below are created from an original image which was a jpg of 72.7kb (865px by 714px). If it was saved as a png the size would be 563kb.
The first image is a png of 35kb (173px by 163px) and uses 'background-size' to stretch the image to the size of the div.
The second image is a jpg of 33kb (1400px by 200px).
If this later image was saved as a png the size would be 277kb (1400px by 200px).
This demonstration is given as a method to use the CSS 3 property 'background-size'. A image will appear in the div below in most browsers and will use either of the above two images depending on browser support for various properties and declarations in CSS 3.