In order to unlock an illustration, many parameters have to be taken into account throughout the episode. On Eldarya, it is the choices during the dialogues, the LoM and your crush that are most often taken into account.
However, other parameters can also be taken into account and the Eldarya team will not communicate about them.
Thus, even if you follow a guide on the Internet, it is possible that you will not unlock the illustration. Your progress is not strictly identical to that of the user whose guide you read.