Notice that Universe is designed to be responsive no matter which sized of a screen you're working with. This means that you cannot fully control how the end user sees the image you've added to Universe - it depends are they on their phones, tablets or desktop computers.
That said, our recommendations for the image sizes are:
Logo in the Universe toolbar: 140 x 26px
Splash image: 1200 x 1400px
Pages header (hero) images: 1920-1024 x 400px
Pages background images: 1920-1024 x 1080-768px
Channel header (hero) images: 1920-1024 x 315px