1024 x 768 is a standard resolution I have seen so far...
If you want to put you image in the background then you don't need to do it through CSS as you can do it with the help of body background property, but if you want to avoid background image tiled on larger screen resolution then you will require to do it with CSS.
If you want to postion the background, then my suggestion is that you keep that background in a table equal to the image size and then put all the page contents in the table. With this you will be able to position the table and the image will positioned with it.
In simple words use the image as the background of table as compare to the page.