Advertizing stuff like trailers or production photos from press kits aren't "leaks", it's marketing.
They release that stuff on social media under a fake name and call it a "leak", expecting it to make the rounds on the internet.
REAL leaks would be footage from filming, clips from the final product, or scripts, and Marvel wouldn't be giving a joking "oh shucks, Hydra!". The corporate giant would use its money and government influence to co-opt anti-terrorism laws in order to avoid due-process again as they hunt down the leaker and copies, shut down sites, and grind the offenders into paste with their pack of attack lawyers and Homeland Security lapdogs.