To learn exactly what MediaTracker does when you call its methods you can look up the methods in the MediaTracker api and read about them in the Method Detail section and/or look in the MediaTracker source code available in your sdk <JAVA_HOME>src/java/awt (after you unzip the src folder, of course).
I made up a small app to show what happens when you load with each method. You can experiment with it to learn how things work.
If you enjoy these things you can also experiment with the ImageObserver interface which is implemented by all Components. So all you need to do is override the imageUpdate method within your image–loading component and you have access to the loading process via flags described in the ImageObserver api.