The two simplest ways are for you to check to see if the image is loaded
 or wait for it to load.  If that is not workable and you need to be
notified, override ImageObserver.imageUpdate() to call a method or fire
an event when the image is completely loaded.

Why exactly do you need to know that the images have loaded?  There
might be some simpler way to handle your problem.

As Knute alluded there are probably better ways of doing this.

First, all images are already loaded asynchronously.  If you use
getImage() it returns immediately, and loads the image in the background.

URL myImageUrl = new URL( "http://myserver/images/myImage.jpg" );
Image myImage = java.awt.Toolkit.getImage( myImageUrl );

The java.awt.Component class implements the ImageObserver interface.
That means all your components (JButtons, Applets, JPanes, JPanels,
etc.) can act as their own image observer. The default behavior is to
call your components repaint() method if the image does not get fully
drawn.  There's a default limit of 100 milliseconds in between calls so
that image loading doesn't completely consume all CPU cycles.

From inside some random component that has a custom image,

public void paint( Graphics g ) {
  g.drawImage( myImage, x, y, this );
  //...
}

will automatically invoke this default behavior.

NOTE 1: Nothing syntax checked.

NOTE 2: From _Learning Java_ by O'Reilly, 3rd ed., pages 709 - 710.  Get
it. Buy it. Love it.