From the Event Home Page, click on Call for Papers
You have only applied branding to your landing page. On the branding page under the Call for Papers, scroll down to configure the login page and CFP form.
We recommend something around 1200 x 300.
Lineup Ninja will resize whatever image you upload so that it fills the full width of the page. This means that if you use a square image, it will look HUGE! We recommend creating a blank file with the background colour that you want, around 1200 pixel wide x 300 pixels high, and paste your logo onto it.