Importing Images: Default "incorrect" scaling of images

[Jay Smith <jay JaySmith com>]

I am a new user: 0.98+Git on Ubuntu Linux 8.0.4 (Hardy).

(In my last post I say I was on 97, but I am on 98, just installed 
today. However, the version information both in the 'About' and by "dia 
--version" is showing 97 ????)
I think I am misunderstanding Dia's concept of importing images or 
missing some important feature???
I have a library of existing 300 dpi TIFF images which I wish to use in 
my diagrams.  The primary output target is high resolution printing, 
thus the 300 dpi is preferable and they already exist in this form.
When I import an image, it is coming in tiny!!  When I print the page 
from Dia, the imported image is about 20 mm wide.  When I print that 
_same_ image from an image program such as Gimp, it prints 151 mm wide.
I understand I can drag it to whatever size I want, but I don't want to 
do that.  I would like to have the image come in to the page at 100% 
"native resolution".
In the object properties, I see that I can keep/constrain (or not) the 
aspect ratio (when dragging larger).  That's great.
However, I don't see anything in the Properties (including double 
clicking on the image item in the tool box) or Preferences about import 
scaling.
I understand that import scaling is complicated because it gets into the 
issue of what the ppi is of the imported image vs the page width however 
that is measured.  However, I don't see any controls available (other 
than dragging) for controlling and manipulating scaling, either upon 
import or after.  For example, it would be really nice to be able to 
numerically define the target width and/or height (depending if 'keep 
aspect ratio' is on or off) both numerically (by a variety of 
measurement methods (inches, cm, mm, etc.)) OR by percentage of original 
native size.
===

I know that this is really for a different thread, but related to this 
issue because it relates to being able to see how large the images are. 
 I can't make any sense of the rulers vs the display window size vs the 
display percentage.  I have changed the Preferences, Length Unit to 
various settings and I can't see a difference in the rulers, either 
dynamically, or when I create a new document, or when I close the 
program and then re-open and create a new document.  From the program 
itself, I can't tell what measurement system the rulers are using.  I 
apparently am clueless about how the rulers are supposed to be used.
And, the "Help, Help" menu item says "Could not open help directory:
Error opening directory '/usr/share/dia/help': No such file or 
directory" so I can't look there for info.  I eventually found
http://dia-installer.de/doc/en/index.html but with no help from the program.

Looking at
http://dia-installer.de/doc/en/canvas-chapter.html#Rulers
says the rulers are in centimeters and measure the size of the "canvas". 
However, I have not found a definition of "canvas" -- is that the size 
of the paper (within the blue lines), or the size of the paper less the 
margins?  Or something else?
I have the page size set to American Letter (8.5 x 11 inches) which is 
about 215 x 279 mm.  On screen, the proportions look right.  But, the 
ruler says that it is about 16.2 cm (162 mm) wide by about 22.8 cm (228 
mm) high.  Those measurements are vastly different than anything I can 
come up with for American Letter.
What am I not understanding?