So, after 30 days of experience I can relay a few things:

1. you only have until payout to fix any typos (7 days from the original time of posting).
2. you cannot insert extra pictures after posting through the regular link underneath the text field. You CAN however insert pictures through html, you can also swap html addresses in the post, and swap pictures out. this will even update in people's newsfeeds, so you could actually swap the thumbnail that way if - in the end after posting - you are not quite happy with what you chose originally.
3. Editing posts costs you bandwidth. So it's wise to take care when editing, and correct as many mistakes at once as you can, instead of updating one fixed mistake at a time.

So far nobody has had an issue with me fixing typos and stuff. I would not try to add whole paragraphs after posting, it should be the final text when posting to steemit. After all - people voted on it, and if you change the ideas within your post LATER, it might come off as a little dishonest.

That said, I always welcome when people go over their text a few extra times and take out all the typos they can find. Seems much more professional, and readers will be able to tell that you put great efforts into your post when there aren't many hang-ups and typos, which will ultimately rewarded by quality-lovers.

Good luck <3