It is important when resizing the images to keep the same aspect ratio.
 In other words, an image that is 200 x 100 can be resized to 100 x 50
(50% ratio on both sides) without distorting the image, but try to
resize to 100 x 80 and the image will be distorted and may be
"bitmappy" or have rough edges when printed.
Different software packages use different methods for doing this, some
require holding down the shift or control key while dragging the side
of the image.  Some require that you drag from the corner of the image.
 Others have a "keep aspect ratio" button to check.