To add a banner at the top on wordpress, log into the dashboard, select appearances on the left and then click editor, then select on the right hand side the file header.php and then paste your code at the end of the file just before the last line. You will really need to know what you are doing depending on the css rules for the header as it may be set at a certain height. To change this go to the very bottom on the right and you should see a file called stylesheet (style.css) inside this file you will probably see #header and there will be lines below this explaining the height which you will need to increase.

Hope this helps
David