You can do this using free software such as Gimp or Paint.NET. On a Windows machine, you can use the inbuilt Paint software to resizeimages. On Macs, you can use iPhoto.
If you need to tweak an imagesize slightly after uploading, it's easy.
In the second tab "Appearance" change the image dimensions manually (this is more accurate)
NB: Super hint! If you wish an image to resize automatically to the size available on the screen, then remove the height property in the image dimensions, and add 100% (or whatever percentage) to the width property! The image will then scale / resize automatically!
Resize this window and see the difference between these two pictures. One will resize, the other will not: