Change image in code with Xamarin.Forms - xamarin.forms

Question

I've tried a few different methods based on the official documentation and from debugging my if/else condition is firing correctly, but I can't seem to get my image to change.
As you can see, I've tried two methods here but my understanding is that ImageSource.FromFile isn't needed because of implicit conversion.
Here's the XAML:
<TableView Intent="Settings">
<TableRoot>
<TableSection Title="Getting Started">
<ViewCell>
<StackLayout Orientation="Horizontal" Padding="10" >
<Image Source="bulb.png" x:Name="imgBulb" />
<Switch x:Name="bulbSwitch" IsToggled="false" Toggled="bulbSwitchToggled" HorizontalOptions="EndAndExpand" VerticalOptions="Center" />
</StackLayout>
</ViewCell>
</TableSection>
<TableSection Title="Time to slide....">
<ViewCell>
<StackLayout Orientation="Vertical" Padding="10" >
<Slider Maximum="100" Minimum="0" Value="50" ValueChanged="sliderChanged" ></Slider>
<Label x:Name="lblSliderOutput" HorizontalOptions="CenterAndExpand" Text="Placeholder" />
</StackLayout>
</ViewCell>
</TableSection>
</TableRoot>
</TableView>
And the code behind:
public partial class TableViewsPage : ContentPage
{
public TableViewsPage ()
{
InitializeComponent ();
lblSliderOutput.Text = "50";
}
public void sliderChanged(object sender, ValueChangedEventArgs args) {
var simpleValue = Convert.ToInt32 (args.NewValue);
lblSliderOutput.Text = simpleValue.ToString();
}
public void bulbSwitchToggled(object sender, ToggledEventArgs args) {
if (imgBulb.Source == ImageSource.FromFile("bulb.png"))
{
imgBulb.Source = ImageSource.FromFile ("biggerBulb.png");
Debug.WriteLine ("imgbulb = bulb.png");
}
else
{
imgBulb.Source = "bulb.png";
Debug.WriteLine ("imgbulb = biggerBulb.png");
}
}
}