getImage(). It takes a finite amount of time for the image to be read. The larger the image the longer it takes.
If the image is loaded from the Internet instead of a local file,
it can take even longer.
Instead of waiting for the image to finish loading getImage() returns immediately, possibly before it's even connected to the remote site.
It does return an Image object, but that Image object may not initially contain any data. All the different methods in the AWT that use images understand this and behave accordingly. For example, if only the top half of an image is available when drawImage() is called, then only the top half is drawn.
Meanwhile, in the background, in a separate thread, an ImageProducer object, that is a instance of a class which implements java.awt.ImageProducer, is filling the image with pixels as fast as it can. When more pixels become available, at least a scanline's worth, the ImageProducer notifies all ImageObserver objects that have registered an interest in this image.
How does an ImageObserver register its interest in an image?
By being passed to a method that does something with the image. For example recall that both drawImage(), getWidth(), and getHeight() take ImageObservers as arguments.
java.awt.Component implements the ImageObserver interface so all its subclasses do too. This means buttons, TextAreas, check boxes, panels, applets, and more can all be used as ImageObservers. However in general you'll use the component that's actually trying to draw the image.
The ImageObserver interface declares a single method, imageUpdate():
public abstract boolean imageUpdate(Image img, int infoflags,
int x, int y, int width, int height)
ImageProducer uses this method to tell the ImageObserver that more data is available for the image, so it should be redrawn.