This paper focuses on difficulties students have in reading images, regarding the understanding of a particular domain, e.g. optics and colour. We concentrated upon what extent teachers were conscious of such difficulties and what they suggested doing to avoid possible pitfalls in this respect. A list of expected obstacles, set up to initiate the study, was shown to be appropriate. It shows that, when they predicted or observed a misunderstanding, the consulted teachers tended to add some elements to a misleading image rather than discussing the symbols that were used, their more or less realistic status and their possible ambiguity. Implications for teacher training and the design of innovative sequences are briefly discussed.