Is this a bug or is the image just hidden in the post and not in the post history?
It's not a bug. Images posted by Newbies are hidden in their posts because of (NSFW?) image spam in the past. If someone chooses to check their post history, it's okay to show the images.
Note that you can already create IMG-tags, and once you become Jr. Member, the images in your older posts show up too.