The Image, or Great Image (大象, dàxiàng), is a one- or two-sentence commentary on each hexagram. It looks at the two trigram figures as images from nature and draws a lesson from the scene: what a wise person would do when facing something like this.
How it's built
It nearly always follows the same pattern: first the scene, then the hexagram's name, then the advice, which begins "so the noble person…" Sometimes the model is the ancient kings or the sovereign instead.
The Image of Activity reads 天行健，君子以自強不息: heaven moves with strength, and so the noble person keeps growing stronger without pause. The Image of Water shows water arriving again and again, and asks for steadiness in virtue and in teaching.
In your reading
The Image is a good partner to the judgment. The judgment tells you how things stand; the Image tells you what attitude to take. If your cast has no moving lines, read it slowly, because it gives you something concrete to do.
Origins
The Image isn't part of the oldest text. It belongs to the Xiang zhuan, one of the commentaries in the Ten Wings, which also includes the Small Image for each line. Tradition credits it to Confucius; scholars date it centuries later, somewhere between the Warring States period and the early Han dynasty.