Body image is an important dimension of life, and in a media-saturated world, we are supposedly surrounded by images of 'idealised' physical body types that are harmful to people's body image. Many theories have been put forward to account for the growing idealisation of 'thin' and 'toned' women and 'lean' and 'muscular' men. Historically, psychological body image research has reliably linked exposure to 'ideal' bodies in mainstream 'traditional' media (along with moderating processes of internalisation, self-comparison, and objectification) and experiences of body image dissatisfaction; for increasingly diverse populations of women and men across the lifespan. In recent years, researchers have turned their attention to social media's impact on body image. Some quantitative and correlational studies suggest that social media use is associated with body image dissatisfaction and, in some instances, further intensifies its scope and scale. Findings also contend that social media can promote body image satisfaction by enabling the distribution and consumption of alternative, 'realistic' body types and body positive content. This thesis suggests inconsistent, and binary findings come from the limited and partial ways research often conceptualise and analyse the relationship between social media and body image. Studies are typically based on singular aspects of social media (i.e. exposure to particular pages or technological affordances) and often overemphasise body image as a static, cross-situational trait, ignoring its complex and situational-specific nature. The current study counters this by bringing together the critical psychological view that body image is complex, multidimensional, and fluid and contemporary Screen and Media scholarship that emphasises how people can simultaneously be users of and used by social media. I combine these two perspectives to critically explore how people experience their body image through social media in ambivalent ways and how social media can enable and challenge the process of developing positive body image experiences. Specifically, I take a Critical Realist informed, qualitative approach to exploring how the social and technological affordances of social media ('Facebook' and 'Instagram') shape attitudinal body image (thoughts, feelings and behaviours relating to research respondents' physical appearance and embodiment) of adult, New Zealand and Australian women and men who are actively using social media to improve their body image. The research findings draw from an online survey of 552 respondents and 30 semi-structured interviews that highlight the simultaneous risks, rewards, and ever-present tensions - which, I argue, are inherently characteristic of the reflexive relationship between particularly situated individuals, social media, and body image.