This is the window that opens when you click on add.

1- click here to select the image you gonna configure

2- the horizontal alignment, you select where the image gonna be(left, center or right)

2.1- the offset, put negative numbers for left, positive for right. With this and 3.1 you can put the image right on the place you want

3- vertical alignment, select where the image gonna stay verticaly(top, center or bottom)

3.1 offset, put negative numbers for up, positive for down.

4- use only if you configuring one of the center images(top or bottom). select it and put 100 and %, so it fills all the empty space in that part.

5- use it if configuring left or right center images. select and put 100 and %.

note: never use both(4 and 5) on a config! It gives bugs...

6- Draw, here you select how the image gonna behave(normal, tile or strech). Use normal for all images, except the center images, where you must use tile. And if you want your background to strech in the size of the skin, select strech. But note that streched images are pretty lame.... they lose all the quality!

7- select here the color of the transparent parts you put on your skin. Don't use invisible unless you made an spacer image..., but I never tryied...

here's an exemple of one config: