By default you can upload one logo for free, which will appear in the center of the of the title bar in the layout that you choose. If you optionally pay to remove the Everwall logo, you can you can upload a second logo in it's place for no additional cost.
The logos you upload will be automatically resized to fit the layout that you choose. In most cases they'll be resized to be no taller than 200 pixels tall, and no wider than 1200 pixels wide. The logos you upload should be 3mb or less, and can be in PNG, JPG or GIF format.