Are you just shrinking them, or are you expanding them?
I would presume that you're shrinking them, and doing so in order to reduce the file size of the document. (Otherwise, most document creation tools allow you to "drag a corner" of an image to simply resize how it's viewed.)
Otherwise, you may want to use a program which would "interpolate" the image in order to reduce "pixellation" of the result, or otherwise apply different filters to the image to achieve other effects.
You can use programs such as "
ImageMagick" or the GNU Image Manipulation Program (
gimp.org) to do such things, including various command-line tools to manipulate images.
http://www.imagemagick.org/www/mogrify.html