I recommend that you look at the skin you took the image from and use the margins that it used. With most things in windowblinds, it's just trial and error until it looks the way you want it. Keep in mind that painting margins just tell it where to stretch or tile the image. The text margins tell it how far from the edge of the image the contents should sit.
And also keep in mind how much space you have between the actual images. I did a little drawing to illustrate. I only drew the button with 2 states instead of 6 like it should have, but you get the idea.
The top image would give you less space in between the buttons than the bottom image
