Workarounds:- I have tested the issue with Unique values instead of the Colormap option, and the grid will display with a transparency. (issue: the user has numerous MXD files, and does not want to go back to each MXD file and change the symbology)- Another workaround for your issue, would be to use the Copy Raster Tool within ArcToolBox. This will create a new Raster image, by making sure you choose the option "Colormap to RGB".