It is quite easy. Find the LS_Global_###x###.cpr file in windows. The #s equal your resolution. Then look at the code below. The image must be the exact resolution for your phone.
If you don't feel like doing this, post your already sized pic and resolution and I will make a cab for you.
Code:
<Form>
<Image ID="Background" Left="0" Top="-52" Width="480" Height="640" Visible="True" Opacity="1.0" ScalingAlgorithm = "TriLinear" Source="Enter Your path.... i.e. \Windows\filename.png"/>
</Form>