To reproduce:

Set an image to the ImagePrimitive of the RadButtonElement of RadButton. Also set the ImageLayout property to Stretch, Zoom or anything else. You will see that this property has no effect.

Workaround:

Use LightVisualElement. This is how it can be used as control:

[ToolboxItem(true)]
public class RadLightVisualButton : RadControl
{
    private RadLightVisualButtonElement buttonElement;


    public RadLightVisualButtonElement ButtonElement
    {
        get
        {
            return buttonElement;
        }
    }


    protected override void CreateChildItems(RadElement parent)
    {
        base.CreateChildItems(parent);


        this.buttonElement = new RadLightVisualButtonElement();
        parent.Children.Add(this.buttonElement);
    }
}


public class RadLightVisualButtonElement : LightVisualElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(LightVisualElement);
        }
    }
}