Skin properties

The skin properties window.

Explaintion of the text fields:

ID: This is the Main ID of the current skin-page. This is like a page number in a book so they should all be unique. ranging from 0 upwards.
XPos: This is the horizontal position of the background image of the skin. (Currently xForm will always read it a 0)
YPos: This is the vertical position of the background image of the skin. (Currently xForm will always read it a 0)
Width: xForm will fill this in for you. But its Image width
Height: xForm will fill this in for you. But its Image hieght.
Default Ctrl: As with the ID of the skin every control on the skin will have an ID. The default ctrl matches up with the ID of a control and will automatically select it in XBMC. Other than this the ID's of the skin and the ID's of the controls bear no resembalance and can even be the same as one another.

Once you browsed for you image and have selected it. xForm will fill in its width and hieght then just click the save button on this window to Update the skin window with you lovely new background.