I assume you mean "1440"X900.?
You can force a smaller size wallpaper to fill the screen by right clicking on your desktop, select properties>Desktop tab. Under "Position" select "Stretch" from the drop down menu.
Unfortunetly, you will end up with a stretched image that does not look real well.
There are plenty of free wallpapers out there available in 1440X900 if you look around.
Another alternative would be to just make your own or take a larger image and crop out a 1440X900 section of it.