From ArcGIS help: An alpha band acts as a transparency mask, providing a transparency value for each pixel. ArcGIS supports alpha bands for the JPEG 2000 and PNG file formats. When you add a raster that contains an alpha band into ArcGIS, it will automatically display the raster dataset with the alpha band turned on. If you wish to display the raster dataset without the alpha band, then you will need to uncheck it in the Symbology tab of the raster layer Properties.