Images not showing

I know this has been asked before, and I've read all the previous threads I can find.

I can get backgrounds and images to show on the emulator fine. I can't get any to show on an Android device connected using LivePlayer.

Images are in Resources/Drawable (also copied in to the hpdi etc. folders)
File properties are AndroidResource, Copy always
File names are lower case and just letters
Image formats are PNG, and all are less than 30K
I've cleaned and rebuilt
I've tried switching between release and debug builds
I've tried with fake image names, and it doesn't throw any errors (e.g. using missing.png, which doesn't exist)
Help!

It used to work, but after lots of unrelated work on the code using the emulator, when I go back to the LivePlayer it doesn't work. I had loads of problems with System.Json working sporadically, and so had to drop that in favour of newtonsoft, but along the way I updated all Nuget packages etc.

<?xml version="1.0" encoding="utf-8" ?>
<TabbedPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:SparkEVX3"
             x:Class="SparkEVX3.MainPage"  BarBackgroundColor="#0e3c63" BarTextColor="Red" >

    <ContentPage BackgroundImage="background.png" Title="Status" >
        <ContentPage.Content>
        <StackLayout  VerticalOptions="Fill" HorizontalOptions="Fill" >
    ...
                </StackLayout>
    </ContentPage.Content>
   </ContentPage>
</TabbedPage>