My New Tumblr Backgrounds come in one size 1680x1050.
If you need a resizer action you can get it here.
Or you can click on the image, Just 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 click here.