An example of an unnecessary comment:
  // incrementing variable x
  ++x;

An example of a helpful comment:
  /* cap pixel values to avoid errors when we
     send it to the frame buffer */

  for(int i = 0; i < img.length; i++)
    if(img[i] > MAX)
      img[i] = MAX;

Finally, you should provide basic documentation for your classes and methods
using javadoc, e.g.: a short description of the method, allowable inputs,
and the output(s). For example:

/**
  Returns an Image object that can then be
  painted on the screen.                                                    

  @param  url      an absolute URL giving
                   the base location of the image

  @param  imgName  the location of the image,
                   relative to the url argument

  @return          the image at the specified URL

*/
public Image getImage(URL url, String imgName)
{
        // code ...
}