Also, in order to increase the resolution of the figure, go to Render Command, in the same panel, and add the flag: -res xxx yyy, defining the number of pixels of the image.
Example:
"/usr/local/lib/vmd/tachyon_LINUXAMD64" -aasamples 12 %s -format TARGA -res 1600 1200 -o %s.tga
Christina