Please try the below XAML snippet:
<Button Width="300" Height="50">
<StackPanel Orientation="Horizontal">
<Image Source="Pictures/img.jpg" Width="20" Height="20"/>
<TextBlock Text="Blablabla" VerticalAlignment="Center" />
</StackPanel>
</Button>
In XAML elements are in a tree structure. So you have to add the child control to its parent control. The below code snippet also works fine. Give a name for your XAML root grid as 'MainGrid'.
Image img = new Image();
img.Source = new BitmapImage(new Uri(@"foo.png"));
StackPanel stackPnl = new StackPanel();
stackPnl.Orientation = Orientation.Horizontal;
stackPnl.Margin = new Thickness(10);
stackPnl.Children.Add(img);
Button btn = new Button();
btn.Content = stackPnl;
MainGrid.Children.Add(btn);