If you go to the Content > Configurations > Site Images, you can see thumb nail images of the Banner Image and Right Sidebar Image (right column image) and see the dimensions -- the Banner Image is 900x129 which means it is 900 pixels wide and 129 pixels high.
The Banner Image must be 900 pixels wide -- and a minimum of 90 pixels high.
The Right Sidebar Image must be 120 pixels wide (or whatever width you have your Right Sidebar width set to at Content > Configurations > Site Layout) -- and can be 100 pixels high to 1000 -- although it's probably good to max the height at 800.