Objects on a Master page are always behind objects on the Body page. FM uses a postscript imaging model (picture a stack of plates with the Master page on the bottom and the Body page on top). There's no way to move a Master page object on top of a Body page object in FM.
If you want a watermark on top of the graphics either do it (manually) on the Body page or use Acrobat's Watermarking tool instead (a lot more control).