You can do this in a few ways...
With Javascript you need to detect what the resolution is with something like this...
http://www.pageresource.com/jscript/jscreen.htm
Then you need some "if" statements which will select a different graphic for 800x600, 640x480 and then an "else" statement for 1024x768 + any other res.
Another option would be to redirect by resolution using php. Either by selecting an include for the graphic based on what the php reads from the browser.
But if I was you, I would create a graphic which is about 700 wide and then stick it inside a table with a background color (inside the table cell) to match the graphic, so you can't see the join. Then set the table width to "100%" instead of specifying a pixel width. Then the page will look filled, no matter what res you view it with.