I hope this question doesn\'t end up being closed for being too broad a subject but I was wondering about Responsive/Adaptive Web Design, i.e. one website for all browsers,
You're correct about the percentages.
As art elements, the habit is to (with CSS) set a percentage width on an image and set it's display to block.
There's a couple techniques for changing the resolution of images based on the client's screen size as well, like responsive-images.js. I don't think there's a perfect answer on how to tackle this issue yet.
If you're really interested in responsive design, I highly recommend Ethan Macotte's Responsive Web Design.