Should be 300x36 or smaller and ideally should have a transparent background.
Should be 200x80 or similar and ideally should have a transparent background.
Should be large enough to fill the background.
Repeat, Attachment and Position
Size
Upload images to lib/layouts/images/custom folder and specify the relative path (lib/layouts/images/custom/mylogo.png). Remember to check the logo under different themes. You may need to clear the browser cache to see any changes you make to the logo here.
Background Property: no-repeat left top fixed
Background Size: cover