How to use "density" parameters

Hello,

I want to print picture with high resolution. I know that the attribute linked
to the resolution is the "density" parameters. But my problem is that I don't
know
how I can use it...

I try different way
image->set(density=>"200")
image->set(density=>"200x200") etc...

But the effect done is that the pictures are smaller, so I have to multiply the
size of the items represented on these pictures... I just want to reduce the
size of one picture pixel, not want to play with the size (i.e make zoom in and
zoom out to look better...)

Is anyone here who know how parameterize this attribute???

Thanks,

Regards
Michael B.