Set BackGround For RadDock

[Mahvash]

Hi,
I have a MainMdi form and a raddock in it, i add a  DocumentContainer to RadDock like this:

Me.RadDockMain.MainDocumentContainer = Me.DocumentContainer1

I want to set backgrouud image for my raddock that display when no child form is open..
i use this code, but when i execute my application in 17" monitors or smaller my image didn't show in stretch mode and lose some part of my image.

Dim im As New ImagePrimitive
im.ImageLayout = ImageLayout.Stretch
im.StretchHorizontally = True
im.StretchVertically = True
im.Image = My.Resources.pic_Main_Image

DocumentContainer1.SplitPanelElement.StretchHorizontally = True
DocumentContainer1.SplitPanelElement.StretchVertically = True
DocumentContainer1.SplitPanelElement.Children.Add(im)

Please help me
Regards,
Mahvash