Once you have added a new image, it will also post to your Timeline that you have updated your cover photo. You might not want your cover photo change to be broadcast on news feeds of the people that like your page.
To remove the cover photo update from your Timeline, hover your mouse over the right hand corner of the new cover photo announcement on your timeline. Click on the icon that looks like a pencil and select “Hide from Page.”
After looking through the Facebook help page, it is impossible to change or upload a cover photo on the Facebook app. So when you get home to your laptop, the dimensions for the cover photo are 851 pixels wide by 315 pixels tall. The alternative is to use the mobile web version instead of the Facebook app to update your cover photo.