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.