Media content matters in social contexts, especially for adolescents who socialize largely with their peers in media(ted) environments. This is well-exemplified by body image development being influenced by media's thin body-ideal and peer influences. This study investigated peer feedback and media imagery interactions through analyzing brain activity using fMRI. Such neural measures complement self-report measures and reveal mechanisms that are unapparent otherwise. Girls (18–19 years-old, N = 24) were exposed to 30 thin and 30 average-sized media models while in an MRI-scanner. They rated each model as 'too thin' or 'normal', followed by congruent or incongruent peer feedback. Results showed increased activity in the ACC and insula in incongruent situations (e.g., participant: 'normal-weight'; feedback: 'too thin'), especially for those with lower self-esteem. This increased activity occurred in brain centers that become active in uncertain situations (e.g., when social norms are exceeded), indicating that media-based feedback directly influenced brain responses to deviating norms.