In Malawi, only 5% of parents state that the right age for a woman to marry is below 18, but 42% of girls get married before they reach that legal age. We document that social image concerns are likely an important mechanism behind that wedge: where the prevalence of child marriage is high, those who do not marry off their under-age daughters are perceived as less altruistic, reciprocal and trustworthy than those who do. We then randomly assign 412 villages to a public donation drive, through which participants could donate maize to be redistributed to the poorest in their village. The idea is that increasing the visibility of charitable behavior - which also contributes to social image - might provide a less costly but as visible alternative to child marriage for parents who are only willing to engage in it out of social image concerns. One year after the intervention, we find that girls' marriages and teenage pregnancies decrease by roughly 30% in treated villages relative to the control group. Consistent with the social image mechanism, (1) charitable behavior increases the most in villages where child marriage was most prevalent at baseline, and (2) in those villages, parents who do not marry off their under-age daughters are no longer perceived as less pro-social than others. We rule out that child marriage is delayed merely because poor families have additional resources due to donations from the drive, and provide evidence that treatment effects increase with the visibility of the intervention. Our findings provide novel evidence on how far individuals might go to protect their social image, and inform new strategies to disrupt arguably inefficient norms when there is a wedge between private and social motives.