[Dvipng] does dvipng include the dpi information in its output?

[jfbu]

I am using dvipng from the TeX Live 2012 distribution, 

This is dvipng 1.14 Copyright 2002-2010 Jan-Ake Larsson

on a mac os x 10.8.2

The installed gs is GPL Ghostscript 9.05 (2012-02-08)

When I use dvipng, the produced images are at 96dpi.

However when I open them in Preview.app (Mac OS X) it tells
me they are at 72dpi.

If instead I do dvipng -D144 for example, then again Preview.app
tells me that the image(s) are at 72dpi.

Hence if I ask preview to display at 100%  (with the option,
 «natural size as will be printed», I don't know the English version)
the displayed image doubles in size, because the number of
pixels per inch is doubled in each direction. 

It is only with dvipng -D72 that the produced image, when
viewed in Preview at its "real size" is indeed on screen with
the dimensions of the original dvi (or cropped dvi, because I use
the -Ttight option, but I tried with and without)

It is as if the png file was missing the "real" dimensions 
information, so Preview just applies its default setting of 72dpi
having as its disposal solely the number of pixels information.

I looked through the documentation and found no hint. 

I must be missing the obvious or perhaps this is a problem with Preview?

If I use gs on a pdf files, the behavior is absolutely coherent. The 
-r option of gs does influence the dots per inch, and this info is
correctly displayed in Preview. The natural size of the image remains
invariant but its number of pixels per inch changes. 

But with dvipng I get the behavior I have described: the natural size
rather than remaining invariant changes with the setting of the -D 
option.

Any help?

Jean-Francois