The best size for the image at the top of your profile is 1920px*360px. For other sizes see here:
These sizes are a guide and will be updated if needed. For users who cannot see the image above:
Cover image: 800*600
Background image: 1920*1080
Profile header: 1920*300
Profile Avatar: 200*200
If you need any help getting your images to fit then email firstname.lastname@example.org - we want to help you look your best!