If you are viewing at 1152x864 and they are viewing at 640x480 what looks normal on your display will look huge on theirs.
Either reduce the dpi you are scanning at or reduce the size of the pic in a drawing package.
I guess you are scanning text docs and saving them as jpg's if not then you should be some software saves text scans as tiff's which is also used by fax software which can cause a problem.
If scanning text use ocr to convert it to a word document much easier to send.
Hope that helps
