Earlier this week Twitter rolled out a new version of their mobile app, AND a new look for your Twitter profile page.
The biggest change you'll notice is the addition of a header image, which may remind you of Facebook's cover photo. You can see it right away if you have the Twitter app - until you upload a header your page will default to the standard charcoal header.
For now, your full size (i.e. Internet) version will remain as is until you add a header.
Here's what the Twitter header image looks like via the web and mobile:
The process of updating your Twitter header image is very simple:
Go to your profile (the "Me" button at the top of the page)
Click the "Edit your Profile" button
On the left click the "Design" link and scroll down to the section to customize your header
For best display, the recommended dimensions are 1200 x 600 pixels