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