This comment has an entirely different meaning when you can see it's in a fanfic thread, but can only see the first few words. ;)

I just added the slash.

Good! Now your fic is artistically complete as well.

Ok, so the goal is to change the src attribute of the <img> tags in setColorScheme, when those <img> tags don't have classes or IDs to refer to them by. So the first thing to do is to add a class to them so we can refer to them by something other than the thing we're changing. We'll do that with calls to the markImagesWithClass function in the $(document).ready function. Then in setColorScheme, find all the images with a particular class, and change their src attribute to point to the right version of the image.

function markImagesWithClass(imageName, className)
{
$("[src*='" + imageName + "']").each(
function (index, img) {
$(img).addClass(className);
}
);
}
function changeImage(imageName, className)
{
$("." + className).each(
function (index, img) {
img.src = imageName;
}
);
}
function setColorScheme(colorScheme)
{
setCookie("colorScheme", colorScheme, 30);
if(colorScheme=="whiteOnBlack")
{
$("body").css({
"background-color": "black",
"color": "white"
});
changeImage("first.png", "firstButton");
changeImage("next.png", "nextButton");
changeImage("prior.png", "priorButton");
changeImage("last.png", "lastButton");
}
else if(colorScheme=="blackOnWhite")
{
$("body").css({
"background-color": "white",
"color": "black"
});
changeImage("firstWhite.png", "firstButton");
changeImage("nextWhite.png", "nextButton");
changeImage("priorWhite.png", "priorButton");
changeImage("lastWhite.png", "lastButton");
}
}
$(document).ready(function()
{
markImagesWithClass("first.png", "firstButton");
markImagesWithClass("next.png", "nextButton");
markImagesWithClass("prior.png", "priorButton");
markImagesWithClass("last.png", "lastButton");
var cookie = getCookie("colorScheme");
if(cookie && cookie != "")
setColorScheme(cookie);
else
setColorScheme(defaultColorScheme);
});

Replace the existing $(document).ready and setColorScheme functions with these. Adjust the file names to match the actual names and paths of your images, and add calls to markImagesWithClass and changeImage for any other images you want to change.