How does the nature of social media content shape the diffusion of protest? This paper provides causal evidence from the 2023 Nahel riots in France, a nationwide wave of unrest triggered by a police killing and propelled by online mobilization. Using data on 537 protest events and more than 50,000 geolocated Instagram interactions, we construct a municipality-by-day panel of social media exposure. Our identification strategy leverages exogenous variation in pre-riot Instagram connectivity—measured via residualized comment flows on a network seeded on football celebrities’ posts—to isolate the diffusion of protest through online ties. We find that digital exposure to protest in connected municipalities significantly increases the probability of local protest, with effects concentrated the following day. Beyond volume, we show that protest diffusion is content-dependent. Pro-movement, emotionally salient, and visually prominent posts are particularly effective at promoting both online engagement and offline action, while exposure to fear-inducing or repressive imagery discourages protest despite increasing attention. Posts with coordination cues (explicit calls to action or protest logistics) have the strongest mobilizing effects. Our findings show that image-based social media accelerate protest diffusion not only by lowering coordination barriers but also by amplifying emotional cues. They reveal how emotional tone, political alignment, and visual structure jointly condition the spread of digital protest.