Cover Photo images should be 1500 x 500 when uploaded.A note though from Lee Smallwood, that the part of the image appears to be just 1500 x 389, so make sure your important elements are within the top 389px.What these changes certainly represent are continued opportunities for businesses to communicate .The profile image that you choose and the cover photo that you upload can go a long way toward communicating to new followers who you are as a business.And your tweets that are getting particularly high engagement (more favorites, retweets, etc.) will actually be displayed with even larger font to bring further attention to them.Do note that the new tweet format on profile pages drops the Expand link that would hae displayed the full tweet along with summary Twitter Card information.
If you have a tweet that is particularly important, you can now pin it to the top of your stream so that users viewing your profile will see it first.
Note that under the cover photo, there are now options for "PHOTOS/VIDEOS" and "FAVORITES" for you or your followers to select.
While many of us will once again have to update our cover photos and profile images, I think these changes represent some very interesting improvements overall.
Like this: Any pinned tweet will immediately replace any previous tweet you might have pinned.
Finally, when viewing a profile, you will be able to select the of tweets you would like to view, whether just tweets, tweets and replies, or tweets with photos and video.