Why a .png converted from a gnuplot postscript output, has transparent background?

Question

I used gnuplot (4.2) to write a graph to a postscript file. But when I convert this .ps file to a .png file using the 'convert file.ps file.png' command, I get a png image which has no background (transparent).

Why does this happen, and how do I get a png image with a white background?