If you have not selected the switch
Use raster image
colour depth in the
Raster graphics part of the dialog, you
have to select a colour depth for all raster graphics here. This is
recommended if, say, you want to export a PDF file for a b&w printer.
Even if your document contains colour images, you should choose
Another sensible application is if the PDF files are only required for screen display. In that case CMYK images are not much use, so select the colour depth