Load image resource from netstandard library in Xamarin Forms (from https://github.com/xamarin/xamarin-forms-samples/tree/master/UserInterface/ThemingDemo)

Example.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
    <DebugType>pdbonly</DebugType>
    <DebugSymbols>true</DebugSymbols>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Xamarin.Forms" Version="4.2.0.709249" />
  </ItemGroup>
  <ItemGroup>
    <Folder Include="Images\" />
  </ItemGroup>
  <ItemGroup>
    <None Remove="Images\ExampleImage.png" />
  </ItemGroup>
  <ItemGroup>
    <EmbeddedResource Include="Images\ExampleImage.png" />
  </ItemGroup>
</Project>

ImageResourceExtension.cs

using System;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace Example
{
    [ContentProperty("Source")]
    public class ImageResourceExtension : IMarkupExtension
    {
        public string Source { get; set; }

        public object ProvideValue(IServiceProvider serviceProvider)
        {
            if (Source == null)
                return null;

            return ImageSource.FromResource(Source);
        }
    }
}

Page.xaml

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:Example"
...
             
...
<Image Source="{local:ImageResource ThemingDemo.Images.ExampleImage}" Aspect="AspectFill" />
...