Using an If statement in javascript to change images

Sorry to ask virtually for a chunk of code, but I seem to always be missing something with javascript (i don't even know java).

First of all i need to get the resolution of a computer with javascript. and then decide which image i should display.

Is a nested if statement possible? and if so, what is the code/syntax?

[this is for the banner of a website, the rest of the website is tables which work on percentage..]

thanks in advance.

Code:

---

//get the screen resolution
var w = screen.width;
var h = screen.height;

if (w == 1024) {
        //nested if
        if (v == 768) {
                //do something
        }
} else {

}

---

Thanks mate. :)