clipped from: researchers.ultraversity.net   
  1. find a nice picture (portrait - not landscape) click the"browse" button to navigate to the picture on your computers hard drive.
  2. choose "100 pixels" for the maximum image dimension and "Good" for the image quality.
  3. hit the "Resize" button. Once your image has been resized, right-click on it and choose save target as. The smaller image will be downloaded to your computer and will be called phpxxxxxxxxx.jpg
Better Quality/Larger Pictures = Larger/Heavier Filesizes = A longer wait for you the Users.

For a really quick and simple 3 step solution try http://www.shrinkpictures.com