Textbooks use images, in addition to text, for delivering knowledge, thereby convey attitudes and values of students including those on gender bias. The gender bias presented in textbook images affects in subtle ways the students' learning outcomes, career choices, and how they perceive science. However, prior research has relied on explicit information presented by textbook images of several subjects to investigate gender representation, overlooking the implicit meaning behind images with a very limited attention to science textbooks. Therefore, this study uses the social semiotic framework to analyse the implicit meaning that images convey related to gender representation in Chinese and Egyptian science textbooks. Specifically, four (two for each country) science textbooks of grade nine were coded and analysed. The findings revealed that gender gap still exists in the images of both Chinese and Egyptian science textbooks. Specifically, females were less represented in the textbook images compared to males, and their role was mostly a caring role. Notably, unlike the Chinese females and the common gender stereotype, Egyptian females were represented in a more active and powerful way compared to males. The findings of this study could help in better designing science textbook images to reduce gender bias.