I scan mine into the computer using 600dpi (come out pretty big) and then I use that basic little Microsoft Paint program to downsize them. If you click on "attributes" it tells you the file size. Then I guesstimate and click on "stretch/skew" and enter a % value to downsize by to get it small enough. I do them in JPEG. Pretty easy to do this using Paint.
Since you can select what size to have the pictures come up, I went about as big as possible and most are about 1000 x 700 pixels or so, then that gives viewers more of a range of size options.
Vaughn