Just to clarify.... If the thread is about your family, a chicken story, an emergency, etc., adding new photos or updates in a new post is okay and won't get you into trouble. It helps readers of an especially-long thread if you at least link to new update posts in the original post to help newcomers to your thread follow the story, so we do recommend adding to the original post when possible.
In the Buy-Sell-Trade area, you can't add any additional information or answers to questions anywhere but in the original post. (Read the Buy-Sell-Trade area guidelines for further information about this).