Links will still count towards the character limit for now as Twitter is concerned with abuse/spam. What we may see at some point is Twitter not counting the first link in a tweet against the 140, but counting subsequent links.

As for images, here's a limit of four per post, and Twittwr groups multi into a display with one URL. Videos, Vines and polls are capped at one.

It's possible to have many, many usernames in a reply, but I don't know if there is an upper bounds.