Since today Twitter allows for a new cover photo to be uploaded.
Twitter recommends 1252x626 pixels and somewhere else also 1500x500p pixels, but when I upload an image of that size, only a part (top) of the image is shown, so some unwanted resizing seems to be done. I also checked this site: http://twitterheadersize.com/, but also the 520×260 recommended by that site does not show the full image in the header.
All I want to upload an image to Twitter and see that exact image as the coverphoto, but somehow that seems to be impossible.