It's not a matter of "allowing" members to make picture-only replies, as the only way to prevent such instances would be a ban. When you post an image, you have both a link and the image tags, which regularly meet the ten character minimum for posts. Like other members have said, people can choose whether or not to abide by the rule, but failing to do so will result in sanctions. You shouldn't need an entire inability to do something to not do it when you know it's against the rules and it'll get you in trouble. This stands for all of the other rule violations too.