Yitian,
Again, it's critically important that you verify not only
the dpi before inserting the image, but whether or not the
image is skewed after resizing it. Setting dpi alone does
nothing to the height and width of the finished image, and
you cannot put more dots (in the real world) in the image
than what you started with. So, again, here are the nuts
and bolts...
Open the image in an image editor, like photoshop, or
paint shop pro. Change the dpi (if needed)to a lower
amount if file size is a concern (72-96 dpi for on-screen
viewing).
Being very careful to prevent skewing, resize the image
now by modifying either the height or width (NOT BOTH)
while making sure the maintain aspect ratio box in the
editor is marked. If it's not, you're going to get a
skewed result. A mark in the box ensures that when one
dimension changes, the other dimension changes by the same
ratio. Set your measuring paramameters to something you
understand, if it's the metric system, use it, if inches,
use that, and so on.
Now, save the image as a jpg with little to no
compression, close, and reopen it. Does it look okay? If
so, you're cool. Now, check the properties, file size,
dpi, etc., and record them. Close the image.
Now, insert into Word using...
Insert | Picture | From File: navigate to the image and
click OK.
Working with images can be quite challenging, and often
requires experimentation to help understanding just what's
happening when you make a change.

jesse