Looks like a window with a semi-transparent background using a PNG image was created
here. If you look at the Splash.cs file in the solution in the zip file you'll find at that link you'll see they use interop to create the effect. Here is a snippet:
public void SelectBitmap(Bitmap bitmap)
{
if (bitmap.PixelFormat != PixelFormat.Format32bppArgb)
{
throw new ApplicationException("The bitmap must be 32bpp with alpha-channel.");
}
IntPtr screenDc = APIHelp.GetDC(IntPtr.Zero);
IntPtr memDc = APIHelp.CreateCompatibleDC(screenDc);
IntPtr hBitmap = IntPtr.Zero;
IntPtr hOldBitmap = IntPtr.Zero;
try
{
hBitmap = bitmap.GetHbitmap(Color.FromArgb(0));
hOldBitmap = APIHelp.SelectObject(memDc, hBitmap);
APIHelp.Size newSize = new APIHelp.Size(bitmap.Width, bitmap.Height);
APIHelp.Point sourceLocation = new APIHelp.Point(0, 0);
APIHelp.Point newLocation = new APIHelp.Point(this.Left, this.Top);
APIHelp.BLENDFUNCTION blend = new APIHelp.BLENDFUNCTION();
blend.BlendOp = APIHelp.AC_SRC_OVER;
blend.BlendFlags = 0;
blend.SourceConstantAlpha = 255;
blend.AlphaFormat = APIHelp.AC_SRC_ALPHA;
APIHelp.UpdateLayeredWindow(Handle, screenDc, ref newLocation, ref newSize,
memDc, ref sourceLocation, 0, ref blend, APIHelp.ULW_ALPHA);
}
finally
{
APIHelp.ReleaseDC(IntPtr.Zero, screenDc);
if (hBitmap != IntPtr.Zero)
{
APIHelp.SelectObject(memDc, hOldBitmap);
APIHelp.DeleteObject(hBitmap);
}
APIHelp.DeleteDC(memDc);
}
}