how to show white symbol in legend of print template

Hi,

I have a map has line features coloured in white. I tried to show the white symbols in the legend of print template ( i changed the legend background to dark grey in the legend.rpx), but because the symbols are brought in as jpeg when priting to PDF, so the background of the symbol has been converted to white and thus ended up with a white line sits on top of white square. My question is, is it possible to have the symbols brought in as PNG (just as in ArcGIS REST api) so that the transperance of symbols could be reserved. Thanks.

regards,

Jack