if you are trying to do it in styling purpose do it in the following way:
<Style x:Key="MyButton" TargetType="Button">
<Setter Property="Width" Value="460"></Setter>
<Setter Property="Background" Value="/Images/AnyImage.png"></Setter>
</Style>
define this in app.xaml or PhoneApplicationPage.resources and in your button control in xaml set your style to MyButton. for example:
<Button style={StaticResource MyButton}/>
if you also want to define width and height for your image you can do it same way by defining style in value section set the value with that style. for example:
<style x:key ="MyImage" TargetType="Image">
<setter property="Width" Value="150"/>
</style>
in my button style define background value ={StaticResource MyImage}