Science textbooks contain a great number and diversity of images that can help students to construct concepts and can, consequently, be interesting tools for science teachers. Students may encounter difficulties in understanding images and it is important to analyse how different features of the images affect the comprehension of the message transmitted by the images. This paper focuses on images of energy that may help to construct key concepts. Documents containing images evoking ideas that are seldom visually explained have been designed, with special attention paid to avoiding possible problems with their use and reading. Interpretations given by students of these documents have been analysed within a theoretical semiotic frame. The results seem to confirm the utility of this frame and have suggested some abilities needed for students to correctly interpret them. Information on teachers' awareness on the students' difficulties when reading documents has been obtained from their comments on students' readings of the images and can provide useful ideas for teacher training courses.