Back in September, Twitter unveiled a pretty significant redesign of user profiles which gave you three different photos to worry about: your profile picture, your header photo, and your background image. The header photo is that giant new option that encompasses your profile pic, and the background image displays on the "whitespace" surrounding your stream.
Once a user uploads these images, they remain constant across all devices - meaning your profile will look the same to someone accessing it on the web or on their iPad.
Today, Twitter has put out a punchy animation that reminds users that it's quick and easy to round out your profile with a header photo, and for god's sake please upload a profile pic because people are tired of looking at that egg.
When Twitter unveiled the new header photos, people immedately began to compare it to the Facebook Timeline. To be honest, the two do share a general look and feel.
Maybe Twitter put out this instructional video because adoption of the header photo is low or at least not where they want it to be. I still see plenty of grey space behind profile pics, now that I think about it.