Size isn’t the only thing that matters when it comes to Twitter headers: quality matters, too. Twitter converts all header and profile images to JPEG files, then compresses the files. While this process conserves bandwidth and helps improve loading speeds, it actually diminishes the quality of the image. This is especially true if you choose an image that has been saved and compressed repeatedly, or if you try to increase the size of a small image by scaling it up.
So, for the best possible header image, choose a photo that has as little prior compression as possible. If you edit your header or profile photo, save it at the highest quality setting. If you have too many colors, smooth gradients, logos, or text, they may look blurry unless they are saved at a high resolution and are the correct size.