Anyway, the Banners are 700x315 and they are in PNG format. To download them just Right Click on the Banner that you want and then Save As.
To use them go to Customize on your Tumblr page then look for "Header" on the left side and click upload.
Now, if you don't have a "Header Upload" feature within your template you will have to add the banner manually and to help you with this just head over to our blog friend, Tumblings.NET for a great tutorial on how to get this done.
If you have any questions you can Contact Me.
My Tumblr Banners are FREE for you to Download and Use
Enjoy. Be creative.