C# checking which imagebox is clicked (onMouseUp)

I want to check which of the 3 image boxes is clicked. I tried using if(leftImage.mouseUp == true) but that does not work and returns a syntax error. So my question is, how do you check if an image is clicked in an if statement.

  public int currentQuestion = 0;

public void ImageCheck ()
    {
       int[] ImageValues = new int[15];
       ImageValues[0] = 1;
       ImageValues[1] = 2;
       ImageValues[2] = 3;
       ImageValues[3] = 4;
       ImageValues[4] = 5;
       ImageValues[5] = 6;
       ImageValues[6] = 7;
       ImageValues[7] = 8;
       ImageValues[8] = 9;
       ImageValues[9] = 10;
       ImageValues[10] = 11;
       ImageValues[11] = 12;
       ImageValues[12] = 13;
       ImageValues[13] = 14;
       ImageValues[14] = 15;

       Random randomize = new Random();
       int initialValue = randomize.Next(0, 4);
       int finalValue = ImageValues[initialValue];


       if(finalValue == 1)
       {
           leftImagebox.Source = new BitmapImage(new Uri(@"projectImages\triangle.png", UriKind.RelativeOrAbsolute));
           midImagebox.Source = new BitmapImage(new Uri(@"projectImages\circle.png", UriKind.RelativeOrAbsolute));
           rightImagebox.Source = new BitmapImage(new Uri(@"projectImages\square.png", UriKind.RelativeOrAbsolute));
           question.Content = "Kan jij de driehoek aanwijzen?";

           vraag.Margin = new Thickness(425, 40, 0, 0);

           aantal.Margin = new Thickness(590, 40, 0, 0);

           question.Margin = new Thickness(205, 112, 0, 0);

           currentQuestion = 1;
       }

       else if(finalValue == 2)
       {
           leftImagebox.Source = new BitmapImage(new Uri(@"projectImages/middelstestaaf.png", UriKind.RelativeOrAbsolute));
           midImagebox.Source = new BitmapImage(new Uri(@"projectImages/kortestaaf.png", UriKind.RelativeOrAbsolute));
           rightImagebox.Source = new BitmapImage(new Uri(@"projectImages/langestaaf.png", UriKind.RelativeOrAbsolute));
           question.Content = "Kan jij de dunste staaf aanwijzen?";

           vraag.Margin = new Thickness(425, 40, 0, 0);

           aantal.Margin = new Thickness(590, 40, 0, 0);

           question.Margin = new Thickness(170, 112, 0, 0);

           midImagebox.Margin = new Thickness(500, 242, 0, 0);

           rightImagebox.Margin = new Thickness(815, 242, 0, 0);

           currentQuestion = 2;
       }

       else if (finalValue == 3)
       {
           leftImagebox.Source = new BitmapImage(new Uri(@"projectImages\triangle.png", UriKind.RelativeOrAbsolute));
           midImagebox.Source = new BitmapImage(new Uri(@"projectImages\square.png", UriKind.RelativeOrAbsolute));
           rightImagebox.Source = new BitmapImage(new Uri(@"projectImages\circle.png", UriKind.RelativeOrAbsolute));
           question.Content = "Kan jij de cirkel aanwijzen?";

           vraag.Margin = new Thickness(435, 40, 0, 0);

           aantal.Margin = new Thickness(600, 40, 0, 0);

           question.Margin = new Thickness(245, 112, 0, 0);

           midImagebox.Margin = new Thickness(470, 242, 0, 0);

           currentQuestion = 3;
       }

    }



   public void inputCheck ()
        //cant seem to work out the code here to check if the imagebox is selected while on the  specific question.                 
      {
        if (currentQuestion == 1 && leftImagebox.MouseUp == true)
        {

        }
      }

   private void leftImagebox_MouseUp(object sender, MouseButtonEventArgs e)
   {

   }
}