This is how I create a graph as a PNG image file with 300 dpi for publication:
ods listing gpath="c:\myGraphsDir" style=journal image_dpi=300;
ods graphics / imagename="myGraph" width=15cm height=10cm;
.... call graphing procs .....
This would create the file c:\myGraphsDir\myGraph.png containing a 300 dpi graph using the journal style which is monochrome. You may also try the style variants journal1, journal2 and journal3.
PG