c#父窗体控制子窗体的问题
有4个form，form1，form2，form3，form4，
form1是父窗体有四个按钮button1，button2，button3，，button4
点击button2，在form1的panel1里面出现form2；
点击button3，在form1的panel1里面出现form3；
点击button4，在form1的panel1里面出现form4；
点击button1，在form2，form3，form4的picturebox1里出现image1
重点是，再次按button2，button3，button4时出现相应的from2，form3，form4，且各form的picturebox1内均有图片image1
[解决办法]
new Form，设置parent为panel，toplevel=false
窗体间传值参考http://www.cnblogs.com/cosoft/archive/2011/08/08/2130659.html
[解决办法]
随便写了下，有点乱，但是应该能实现你的要求

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            FormCollection collection = Application.OpenForms;
            foreach (Form form in collection)
            {                
                if (form.Name == "Form2")
                {
                    ((Form2)form).SetImage();
                    form.Select();
                }
                else if (form.Name == "Form3")
                {
                    ((Form3)form).SetImage();
                    form.Select();
                }
                else if (form.Name == "Form4")
                {
                    ((Form4)form).SetImage();
                    form.Select();
                } 
 
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (!GetFormIsOpen("Form2"))
            {
                Form2 form2 = new Form2();
                form2.TopLevel = false;
                panel1.Controls.Add(form2);
                form2.Show();
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            if (!GetFormIsOpen("Form3"))
            {
                Form3 form3 = new Form3();
                form3.TopLevel = false;
                panel1.Controls.Add(form3);
                form3.Show();
            }
        }

        private void button4_Click(object sender, EventArgs e)
        {
            if (!GetFormIsOpen("Form4"))
            {
                Form4 form4 = new Form4();
                form4.TopLevel = false;
                panel1.Controls.Add(form4);
                form4.Show();
            }
        }

        private bool GetFormIsOpen(string formName)
        {
            bool result = false;

            FormCollection forms = Application.OpenForms;
            foreach (Form frm in forms)
            {
                if (frm.Name == formName)
                { 
 
                    Control[] control = panel1.Controls.Find(formName, false);
                    if (control != null && control.Length > 0)
                    {
                        control[0].BringToFront();
                    }
                    result = true;
                    break;
                }
            }

            return result;
        }
    }
}

然后分别在Form2、Form3、Form4中加入公开方法：

public void SetImage()
        {
            this.pictureBox1.Image = Image.FromFile(@"C:\Users\Public\Pictures\Sample Pictures\back.png");
        }