Your Twitter profile photo is the main image that represents you or your brand across the network. It’s going to be seen across the site in a number of places by a number of people so make sure it’s of the highest quality.
On Your Page: The largest display of your profile picture is on your homepage and can be viewed by your followers as well as individuals who stumble upon your page.
In-Stream: A smaller version of your profile picture appears in a follower’s Twitter stream every time you Tweet. It also appears in the stream of your followers’ followers every time you’re Retweeted.
Who to Follow: Your profile picture is also going to appear next to a link to your page in the “Who to follow” box. This is located directly to the right of your twitter stream. Choose a recognizable image here.
Square Image recommended 400 x 400 pixels.
Maximum file size 2 MB.
Image types include: JPG, GIF or PNG.
Header Photo: 1,500 x 500