Compare http://matplotlib.org/devdocs/examples/api/watermark_text.html and http://matplotlib.org/devdocs/examples/api/watermark_image.html. The text watermark is overlayed on top of the axes -- as one would expect for a watermark. The image watermark is below the axes, which sort of defeats the purpose of a watermark (IMO).

A quick fix could be just to add zorder=10 (or whatever large number) to the call to figimage, in order to draw the watermark on top of everything else. Perhaps a better option (I don't actually know whether that's possible, or whether that would actually make a difference) would be to set the background of the axes to be transparent, so that it is drawn on top of the watermark but the white parts don't hide it -- perhaps the masking behavior of the orange semitransparent markers will differ depending on whether they're drawn below or above?