public partial class MainWindow : Window
    {
        private List<Person> people = new List<Person>();

        public MainWindow()
        {
            InitializeComponent();
            people.Add(new Person { Firstname = "Sinead", Lastname = "McCoy" , Sex = "Female" });
            people.Add(new Person { Firstname = "Andrew", Lastname = "Warner", Sex = "Other" });
            people.Add(new Person { Firstname = "Ailish", Lastname = "Man" , Sex = "Female" });
            people.Add(new Person { Firstname = "Andrew", Lastname = "McCory", Sex = "Other" });
            people.Add(new Person { Firstname = "Andrew", Lastname = "McCall" , Sex = "Male" });


            //people.Add(a);
            cmbMyComboBox.ItemsSource = people;

        }

class Person
{
    private string _sex;
    private string _cmbimage;
    public string Firstname { get; set; }
    public string Lastname { get; set; }
    public string Sex
    {
        get
        {
            var sex = new string[] { "Male", "Female" };

            if (sex.Contains(_sex))
            {
               return _sex;
            }
            else
                _sex = "Other";
                return _sex;
        }

        set
        {
                _sex = value;
        }
    }

    public string Fullname
    {
        get
        {
            return $"{Firstname} {Lastname}";
        }

    }

    public string Cmbimage
    {
        get
        {
            if (_sex == "Male")
            {
                _cmbimage = "C:\Users\richa\Pictures\Icons\WPFdemo\cmbimage.png";
                return _cmbimage;
            }
            if (_sex == "Female")
            {
                _cmbimage = "C:\Users\richa\Pictures\Icons\WPFdemo\cmbimagegirl.png";
                return _cmbimage;
            }
            else
                _cmbimage = "C:\Users\richa\Pictures\Icons\WPFdemo\cmbimageother.png";
                return _cmbimage;
        }
    }

}

<ComboBox x:Name="cmbMyComboBox" Grid.Column="1"
              Grid.Row="3" Grid.ColumnSpan="2" Margin="10">
        <ComboBox.ItemTemplate>
            <DataTemplate>
                <StackPanel Orientation="Horizontal">
                    <Image Source="{Binding Cmbimage}" Height="20" Width="20"/>
                    <TextBlock Text="{Binding Fullname}"/>
                    <!--<TextBlock Text=" "/>
                    <TextBlock Text="{Binding Lastname}"/>
                    <TextBlock Text=" "/>
                    <TextBlock Text="{Binding Sex}"/>-->
                </StackPanel>
            </DataTemplate>
        </ComboBox.ItemTemplate>
    </ComboBox>