Wow! My OCD and/or ADHD usually cause me to catch something like that, but I totally missed it! Good catch by whoever caught it!
I zoomed in on the first pic, and I do not know what they are seeing to make the claim that the shot took place on a billboard. However, the flash does seem to be partly blocked by the tower...which is...odd. Unless that is because the windows on the towers do not appear to be facing the crowd, the shooter is in the tower, and he/she has to lean out of the window to shoot. Then you may not be able to see the see the whole muzzle flash.