Hide an element on the master page

I have an image element on a master page (which I use as a watermark).  Based on some condition, I either want the image visible or hidden.  How do I got about referencing the image to hide it on ALL instances of the master page?

If I do something like this, it only works for the first instance of the master page:

     Form1.pageSet.MasterPage1.Mysubform.watermarkField.presence="hidden";

All subsequent pages which use the master page still have the watermark.