This post is from the RTOWN design team:
As of a few days ago, Twitter profile owners will now be asked if they want to switch over to the new layout. This will change the way Twitter profiles are viewed and cause most current profiles to look distorted and unprofessional. The change is making Twitter a much more visual medium and will present another great opportunity for messaging.
Backgrounds have stayed the same, but are no longer visible on the 'me' page, they're on 'home' page
Cover images have increased from 260px x 520px to 1500px x 500px to become a responsive design (they're trying to phase away from the app). So all cover images need to be 1500 x 500 as standard.
Profile photos have gone from 73 x 73 to 400 x 400 as standard.
Curious about how we can now take advantage of the new Twitter layout. Give this Miami Harold article a read
Need more information about the change over? Read Mashable's latest article.