Images

Here is a JPG http://www.flickr.com/photos/splat/49792783/in/set-981332/ at the main link.
Well, it's a picture of a whole site.

Here is a Gig http://www.cwsomaha.com/templates/cwsomaha.com/images/widget.gif
It's from site http://www.cwsomaha.com/

This site has some nice ones.
http://emotionslive.co.uk

http://emotionslive.co.uk/portfolio/realsize/20060609003709.jpg
http://emotionslive.co.uk/img/logo.gif

I couldn't find a png at the sites you mentioned but i did find one at Apple.
http://images.apple.com/macmini/images/features_hero20090303.png
http://www.apple.com/macmini/features.html

It seems that 99% of the images are jpgs