Something to keep in mind is that when I say the blockchain is a form of creative commons, that is not inclusive of images and video in your post. Images are stored by a 3rd party hosting solution (I believe Amazon currently) and of course video is mostly youtube. I mention this both because it seems to be a common misunderstanding (e.g. comments along the line of "this is on the blockchain forever now" directed at an image or video) and also because it is precisely the images that are most relevant to your original post.

So what is stored on the blockchain (i.e. in steem full nodes run by witnesses)? All the text in your posts, as well as a record of all transactions (comments, upvotes, edits, steem transfers, etc.).

That is what is in the Steemit creative commons - and my understanding of the public nature of the Steemit blockchain is that any 3rd party can build on top of the blockchain in all manner of different ways - this to me is a direct analog of taking someones picture and modifying it - but it is on a different scale and in a very different medium :)