Paul_o_b - 3:55 pm on Aug 29, 2011 (gmt 0)
The first value is horizontal and the second value is vertical. So "center 200px" will be the same as "50% 200px".
Which means that the point that is 50% along the horizontal axis of the image will be placed at the 50% horizontal position of the element it is placed in thus aligning it horizontally central. The second value "200px" is the vertical and will place the image down from the top of the element by 200px.
When using keywords only then browsers can work out the position so it doesn't matter what order you use them in (e.g. "top left" or "left top" will both work) but it is still better to state them with horizontal fist followed by vertical.
You do not need to add transparent (unless over-riding a previous setting) because that is the default and will just be like stating the obvious :)