To change the logo of your Page to a customized image, hover over the logo in the editor of your portfolio. You will see a pencil icon appear - click on the pencil and hit "customize"
In the Logo Container pop-up click on Image tab (make sure to click Site Title and that it is toggled on) and choose the image the you want to use as your logo. Click upload and it will load. When it is done loading, click "Done" and you should be all set!