Also, do you have access to PHP?
If you do then you could do the image selection serverside with a method like...
In the head of the HTML page
$img = array(0=>"Sunday.gif",1=>"Monday.gif",2=>"",3=>"Wednesday.gif",4=>"Thursday.gif",5=>"Friday.gif",6=>"Saturday.gif");
$dayOfWeek = Date("w"); // 0=sunday
$theImage = $img[ $dayOfWeek ];
then in the body where you want the DOW image
<img name="DayOfWeek" src="./image/path/to/folder/<?php echo $theImage; ?>" />
*not tested and not guaranteed to work but the principle is to gather the DOW, assuming that the web host's server has the correct time / date.
Anyway, thats the idea floated if you want to do the same server-side as opposed to client-side, both have advantages and disadvantages, one already discussed and also another is if the server and client are in different timezones or the clients clock is not set correctly, yes, some people can't be bothered to have the right time on their computers, go figure!!
So if someone wants to add to that, point out any errors, be my guest.