Quick Tips Needed : Full browser image

As topic say:

What is the best way for a full browser image.

Size format:
The biggest?
The smallest?

Load/Attach, as in
Type: swf/jpg/png

I am using the onResize listener.

But i do got some Questions as in above.
If you choose for example the smallest user resolution wich is 800x600.
The pict looks neat enough for that user.
If you choose for example the 2048*1838 the picture looks neat for that user.

If the picture enlarge in a higher resolution, you got quality loose.
If the picture is large enough, and you reduce your picture size for the smaller resolution user you got a weird look too:)

Somebody with some good tips?

Or is the best solution for this. A real browser resolution check. and than go further.
Or an if and else on the user System.capabilities.screenResolutionX and Y ?

Thnx in advanced