How can I prevent a reduction of color depth?

[pvpp]

I've got a original scan-picture with 48 bit color depth, these should turned and cropped.
But after processing I've got as result a picture with 24 bit instead of 48 bit color depth.
How can I prevent this?

pvpp

[xnview]

XnView only use 8bits per component as internal data

[pvpp]

OK, thanks

pvpp