I have made a mistake in mistaking Javascript for Java Script. They are two completely different things. Javascript is a completely different way of programming which we are not going to learn. Java script is what we are learning. It is the text that you type in order to program in Java.

A java applet is something like an animation or something else that you may put on a website. You can interact with that applet in some way. You do not have to make your own applet to be able to use one. You may find applets in many different places on the internet and use them (as long as it is legal to do so).

An applet is a small piece of executable code that needs a full application to contain it. I have seen a few examples of it being used to modify images. I have seen one that made a water reflection shimmer by putting an "applet code" in the code of the applet which was programmed to make the bottom of the image shimmer.
They made a class called lake.CLASS and put that in the same directory as the page that they are using the applet on. You may put the image file anywhere on the server but you must tell that applet where the image is located.