You can split text up into individual lines by using the new line escape sequence, '\n'. For example
The GreenfootImage constructor you are using will create the image accordingly, centering each line within the image.
String longTextLine = "I want that this line be broken up into several different lines when printed to the terminal"; // could be String longTextLine = "I want that this line be\nbroken up into several different lines\nwhen printed to the terminal";