codebase for the applet is /ntnujava/ejs/
so all the directory are relative to http://www.phy.ntnu.edu.tw/ntnujava/ejs/
so you will find those two images at
http://www.phy.ntnu.edu.tw/ntnujava/ejs/_examples/_data/dog.gif
http://www.phy.ntnu.edu.tw/ntnujava/ejs/_examples/_data/dogy.gif

Think about the file structure and you should be able to figure it out by yourself later on.