An image also helps to break the monotonous exam preparation, understand a Java concept better and retain facts about it in the exam (when you need them the most).
I have used multiple images in my book 'OCA Java SE 8 Programmer I Certification Guide' to help you prepare for this exam.
A few examples:
-----------Image 1 : The 'main' method ----------
The following image will help you to understand and remember components of the main method:
-----------Image 2 : Difference in object reference and primitive reference variables, when passed as parameters to a method ----------
In the following image, the (irritated) girl represents an object reference variable and the (cool) boy represents a primitive variable. It will help you to remember that they are treated differently when passed as arguments to a method:
-----------Image 3 : Assigning a long value to a variable of type int ----------
In the following image, a person is forcefully trying to store a 'long' value in a container that is capable of storing an 'int' value. It will help you to remember that if you do so, the Java compiler will complain. When done forcefully (with an explicit cast), you'll lose information: