Changing 2011 theme header image based on user dropdown

I had to take information from both of your posts to be able to come with something.

Try replacing

<img src="https://wordpress.stackexchange.com/questions/89211/<?php header_image(); ?>" width="<?php echo $header_image_width; ?>" height="<?php echo $header_image_height; ?>" alt="" />

width

<?php
if (!(isset($_POST["styles"]))) { //if post not set (first visit)
    if ($_COOKIE["chosenStyle"] == "night") { 
        echo '<img src="'.get_template_directory_uri() . '/images/headers/SomethingOriginalTheMoonAndStars.png" width="1000" height="288" alt="Night Header" />';
    } else {
        echo '<img src="'.get_template_directory_uri() . '/images/headers/SomethingOriginalSun.png" width="1000" height="288" alt="Day Header" />';
    }
} else { //if post is set 
    if ($_POST["styles"] == "day") { 
        echo '<img src="'.get_template_directory_uri() . '/images/headers/SomethingOriginalSun.png" width="1000" height="288" alt="Day Header" />';
    } else {
        echo '<img src="'.get_template_directory_uri() . '/images/headers/SomethingOriginalTheMoonAndStars.png" width="1000" height="288" alt="Night Header" />';
    }   
}
?>

just remove you setNightHeader() and setDayHeader() functions