Chemistry uses a diverse range of images to build its knowledge, yet the role of images in building complex Chemistry knowledge remains underexplored. This chapter explores how images express complexity. Specifically, it focuses on changes in the images used through years of secondary school textbooks in New South Wales, Australia. We draw on the concept of ‘semantic density’ (more specifically ‘epistemic–semantic density’) from Legitimation Code Theory to examine the complexity of epistemological meanings presented in image. We outline a ‘translation device’ for enacting this concept to analyze images in our data. Our analysis suggests that through the curriculum stages of secondary school Chemistry, the knowledge expressed by images does not simply grow in complexity but represents a growing range of different levels of complexity. The images used by secondary school Chemistry textbooks typically maintain a connection to the everyday world but reach further and further into greater complexity from this base. We conclude by considering what role this growing semantic density range might play in building knowledge through secondary schooling.