问Kivy图片变成白色，请问为什么？

<MyApp>:
    canvas.before:
        Rectangle:
            pos: self.pos
            size: self.size
            source: 'files\background.jpeg'
    orientation: "vertical"
    ScreenManager:
        id: screen_manager
        GuideScreen:
            name: "guide"
        InteractiveScreen:
            name: "interactive"
        WorkshopScreen:
            name: "workshop"
        ShopScreen:
            name: "shop"
        ProfileScreen:
            name: "profile"
    BoxLayout:
        orientation: "horizontal"
        size_hint_y: None
        height: dp(50)
        Button:
            text: "导游"
            on_press: screen_manager.current = "guide"
        Button:
            text: "互动"
            on_press: screen_manager.current = "interactive"
        Button:
            text: "创意工坊"
            on_press: screen_manager.current = "workshop"
        Button:
            text: "商店"
            on_press: screen_manager.current = "shop"
        Button:
            text: "个人信息"
            on_press: screen_manager.current = "profile"


<GuideScreen>:
    Label:
        text: "画游千里江山！"
        font_size: 24
        size_hint_y: None
        height: self.texture_size[1] + dp(10)

<InteractiveScreen>:
    Label:
        text: "互动"
        font_size: 20
        size_hint_y: None
        height: self.texture_size[1] + dp(10)

<WorkshopScreen>:
    Label:
        text: "创意工坊"
        font_size: 20
        size_hint_y: None
        height: self.texture_size[1] + dp(10)

<ShopScreen>:
    Label:
        text: "商店"
        font_size: 20
        size_hint_y: None
        height: self.texture_size[1] + dp(10)


<ProfileScreen>:
    Label:
        text: "个人信息"
        font_size: 20
        size_hint_y: None
        height: self.texture_size[1] + dp(10)