The white and grey chequerboard is the on-screen preview since a flat colour might get confused with part of the image (it's the same effect as Photoshop uses). When saved as a tiff or png file (both of which should be a bit smaller than the bmp, without actually losing any data) you will either literally see through the image in those areas to whatever is behind or you will get a flat colour that probably depends on the viewer used. When opened in an editor you will get a partially transparent image or you will get your render on black with a strange border and the alpha channel will be available as a saved selection that can be used to float the image, allowing the insertion of a new backdrop or further image layers.