The Small Image (小象, xiǎoxiàng) is a very brief commentary, usually a single sentence, attached to the line text of each line. If the line text says what's happening, the Small Image says why.
What it sounds like
It usually repeats a few words of the line text and adds the reason. On the first line of Activity, the text says "hidden dragon, do not act." The Small Image adds: 陽在下也, "because the yang is at the bottom." The strength is there, but it's still down low, with no room to show itself.
Most of its reasons come from the line positions: whether the line is central, whether it's correct, whether it has a partner that answers it, whether it's firm or yielding.
In your reading
When you get a moving line and its text feels obscure, the Small Image can give you the key in a few words. It reads like a margin note from an old teacher.
Origins
Together with the image (the Great Image), it makes up the Xiang zhuan, which fills two of the Ten Wings. It's called "small" to set it apart from the image of the whole hexagram. Tradition credits these commentaries to Confucius; scholars date them to the following centuries, around the Warring States period or the early Han dynasty.