Hi Jean! You MacDudette you! (actually... the women I grew up with in California prefer the term Dudess....

)
Although the email trick I just explained works fast & fine....I work in the program 'Canvas'....where I can open the "image" menu and change the pixel size and/or the resolution.
Ok...this is my normal procedure. I scan the piece (or bring it in from the digi-cam) at a high rez....at least 300dpi.....then I go to 'image mode' , choose 'area' ...then change the resolution to 100 and the pixel size to within the limit of what's required of us here (400x600)...and do a "save as" jpg. That's it.
Of course, this does not guarantee that it will always be at our precendent 100kb ....but, 90% of the time it does it for me.
-Gear