Alternatively, you can change the picture through scripting like you thought. The script of the page where you have the map would be something like this:
IF %SECRETVARIABLE = 1 THEN
$PAGETEXT := $PAGETEXT + "<img src='http://chooseyourstory.com/resources/images/user/16013'/>"
ELSE IF %SECRETVARIABLE = 2 THEN
$PAGETEXT := $PAGETEXT + "<img src='http://chooseyourstory.com/resources/images/user/16014'/>"
The bolded part is number the site gives your picture. Go to My Pictures, and either hover over the picture (the bottom left of your screen might tell you the number), or click on the picture and it will try to download it, with the file name as the number.
I got the basic info from an article Zakari made before: http://chooseyourstory.com/help/articles/article.aspx?ArticleId=108
So yes, it's possible to do it with scripting and it isn't too hard.