If you want to decrease the size you can download my Tumblr Resizer for Photoshop Here.
To Download the Backgrounds just click on the image you want , Right click and Save As.
Preview opens at 1680x1050.
Make sure you change the following line in your Tumblr Custom HTML: top left fixed repeat to this top left fixed no-repeat.
If you need help with adding a background Follow this link.
As Always, they are free for you to Download