Alphamask is a display tool. Your 'OverlayMask' variable is a scalar because alphamask returns a handle to the graphic. If you wanted the logical matrix, you already have it as 'ContornoMask', right?
To save the image with the mask applied, I recommend saveas:
saveas(OverlayMask, 'imagefile', 'jpg')
Or, in general, gcf can be used as a handle to the figure if you like:
saveas(gcf, 'imagefile', 'jpg')
Of course you may choose different file formats including Matlab's 'fig'. If you find alphamask useful, please leave a rating on the file exchange page.