This sample shows how to create a splash screen using Monodelop and Gtk#. Create a new Window. Within its properties, unset the Decorator so that it won’t have any title bar. Add a HBox or VBox inside the window with only one placeholder. Add a button inside and select Custom in the Button Type under the Button Properties. Add an image inside the button.
Source code that makes it work:

using System;
using System.Threading;

namespace sigaDesktopClient
{
	public partial class MainWindow : Gtk.Window
	{
		
		public MainWindow() : 
				base(Gtk.WindowType.Toplevel)
		{
			this.Build();
			ThreadStart tStart = new ThreadStart(this.EndSplash);
			Thread t = new Thread(tStart);
			t.Start();            
		}

		public void EndSplash()
		{
			Thread.Sleep(2000);
			Gtk.Application.Invoke(
	                delegate (object sender, EventArgs args)
                    {
                         StartApplication();
                    }
			);
		}
		
		private void StartApplication()
		{
                        // WRITE THE CODE TO OPEN THE MAIN GUI HERE
			this.Hide();						
		}
		
		protected virtual void OnClick (object sender, System.EventArgs e)
		{
			StartApplication();
		}

		protected virtual void OnEntered (object sender, System.EventArgs e)
		{
			StartApplication();
		}
	}
}