Use the PNG driver and set
export GRASS_WIDTH=xxx
export GRASS_HEIGHT=yyywhere xxx and yyy correspond to the raster map size.
Then d.out.file with res=1 to save the map into PNG.
note you can use the d.out.file size= option in place of GRASS_WIDTH and GRASS_HEIGHT.
also a georegistered GeoTIFF is available for output, see:
http://grass.osgeo.org/grass64/manuals/html64_user/d.out.file.html
but generally it is better to export RGB bands as a group to create
a three band GeoTIFF. Pass r.out.gdal an imagery group to do this.
Hamish