Preserve background color ?

[Bingo]

But do you have same result with ImageMagick?

For the moment, I use mogrify, which is one of the softwares that ship with ImageMagick, and the background is pure white.
But - I have no idea why this is so - mogrify is very very slow on large images, while nconvert is very fast ! There really is a LOT of difference : less than 1 minute with nconvert, sometimes 1 hour with mogrify !
That's why I wanted to switch over to nconvert !

[xnview]

But do you have same result with ImageMagick?

For the moment, I use mogrify, which is one of the softwares that ship with ImageMagick, and the background is pure white.
But - I have no idea why this is so - mogrify is very very slow on large images, while nconvert is very fast ! There really is a LOT of difference : less than 1 minute with nconvert, sometimes 1 hour with mogrify !
That's why I wanted to switch over to nconvert !

Could you tell me which command line do you use with mogrify?

[Bingo]

Of course :
mogrify.exe -depth 8 -colors 256 "my16McolorsPNG.png"