Use this dialog box to specify the color that should appear transparent in the image.
Click Select and pick a color on the image. When transparency is turned on, the selected color is transparent.
Setting transparency color is available only if the image is loaded and the Raster Extension is loaded. To load the image, select the image, then right-click the image frame 
. To load the Raster Extension, choose a Raster Extension command.
The transparency color is stored in the drawing as an AutoCAD Map 3D custom object. If you send the drawing to other users, they can see the transparency color only by opening the drawing in AutoCAD Map 3D or Autodesk Raster Design. If they open the drawing in AutoCAD, they see a message that AutoCAD cannot reference the custom object and will not display the transparent color.