Steve said 'If Oswald had information to reveal he had numerous opportunities to do so.' The picture he posted refers to that, not to an opportunity to shoot Oswald.
Aside from that photograph, there's another one of Ruby in the hallway a few feet behind Oswald. If he had been instructed to kill Oswald, that would have been the ideal time to strike since he wouldn't have known if he would ever get another such (relatively-close) encounter.