Xlabs : Imagebutton doesn't show the Icon image

I am using Xlabs ImageButton . The icon inside the image doesn't show in my screen. Xlab.Forms version is 2.0.5679. Here is my code ,

<controls:ImageButton x:Name="TwitterButton" Text="Twitter" BackgroundColor="#01abdf" TextColor="#ffffff"
                                  HeightRequest="75" WidthRequest="175" Orientation="ImageToLeft"
                                  ImageHeightRequest="50" ImageWidthRequest="50" >
                <controls:ImageButton.Source>
                    <OnPlatform x:TypeArguments="ImageSource">
                        <OnPlatform.iOS>
                            <FileImageSource File="icon_twitter.png" />
                        </OnPlatform.iOS>
                        <OnPlatform.Android>
                            <FileImageSource File="appicon.png" />
                        </OnPlatform.Android>
                        <OnPlatform.WinPhone>
                            <FileImageSource File="Images/icon_twitter.png" />
                        </OnPlatform.WinPhone>
                    </OnPlatform>
                </controls:ImageButton.Source>
            </controls:ImageButton>