Pierre,
Now I understand what you were trying to do with 'Keep Document Size'.
I agree with Troken that disabling the height, width and unit editboxes in the 'Print Size' Group Box is a good idea and would let the user know what is happening.
Yes please, change the
default action to initialize the dialogbox with the dimensions of the current image being processed.

Bug 1:
Just to clarify what troken said about his bug.
You can set the DPI to anything you like manually by typing, but the up-down control will not increment past 100.
You need to code something like this...
SendMessage(hWndControl,UDM_SETRANGE,0,MAKELONG(UD_MAXVAL,UD_MINVAL));

Bug 2:
There is a different problem with all your other edit boxes that are attached to up-down controls (not the Units one).
Just say that an editbox is initialized with the number 300.
You can manually change it to another number by typing, say 100.
But if you try to increment that number with the up-down control, it will increment to 301 rather than 101.
Also decreasing is the same.

Half a bug 3:
(I feel bad about giving you these bugs when you are already doing so much).
Troken is right.
If 'Keep Document size' is checked, the print size should not change when you change the screen size.
All the scaling should be done with the DPI.
You can see the the problem easily by doing this:...
Check 'Keep Ratio' and 'Keep Document Size'.
Set screen size to "pixels" and print size to "dots".
If you change either the width or height in the 'Screen Size', the 'Print Size' will change by the same amount.
But I think this is impossible to fix if you are changing the aspect ratio (if 'Keep Ratio is unchecked), because by its nature, changing the ratio would
have to change the print size.
I can only suggest that you can only check 'Keep Document Size' if 'Keep Ratio' is checked.
One question:
I also use C++.
When describing problems, I use English descriptions from MSDN (like Group Box for example).
Is this a help to you or does it make it more difficult because you work with a French version?