The abundance of images concerning textbooks doesn’t always facilitate the understanding of the concepts. In this study, the phases of the moon, the images shown in Primary and Secondary textbooks, the relationships between the written theoretical content and its illustration, as well as the problems of comprehension of some images, are studied. We worked with 78 students in the third year of a uni-versity teacher-training course. The images used to illustrate the phases of the moon are not, by themselves, sufficiently explanatory. Students haven’t a clear idea of the Sun/Earth/Moon model and they lack some concepts with which to build it. They have also difficulty in expressing themselves in diagrams. Very strong inducements to attract attention are required in the written text, as well as specific instructions on the part of the teacher, if learning is to take place.