Looks like you have already figured it out. For those who have not...
The picture at the top of the page is 955 pixels wide by 220 pixels tall. The tops of the lettering of the menu are at about 195 pixels from the top. Use your favorite editor to crop a picture to the desired size. If the picture is of larger dimensions, the excess may not show, depending on settings.
Under your login name at the upper right, choose Settings, then choose My Page, finally, choose Appearance. Click the All Options tab. Assign your picture to Header Image. While there, you will see many additional options that can be selected to affect the appearance of your page.