Similar thread on PCA:
Paint.net is free.
The thing is, do you ever want to print the pictures? If so, 300 dots per inch (dpi) is needed. If you only want to see them on your monitor or iPad, or send them by e mail just to be seen on a PC, then 72 dpi is fine.
Once you have reduced them, however, unless you keep the original 300 dpi, the smaller ones will be blurred if you try to print them.