It's been confirmed that Ultron will take many forms over the course of the film, evolving and upgrading himself until he reaches his final form. With that in mind, I thought it would be helpful to take a look at his progression, based on what we've seen, as well as a look at his final form courtesy of a new piece of concept art!
Ultron will likely look like this at the start:
Before upgrading to look more like this:
Before finally settling on something sufficiently creepier like this: