How can i draw PNG in my dialog on demand?

[I_want_to_lean]

I know how to draw PNG in my dialog using OnPaint() but i don't know how to draw a PNG after i click a button.

Here's the code for the OnPaint() that i use

#include <gdiplus.h>

#pragma comment (lib,"gdiplus.lib")

using namespace Gdiplus;

... all MFC stuff

void CAppName::OnPaint()
{
       CPaintDC dc(this); // device context for painting

	if (IsIconic())
	{

		SendMessage(WM_ICONERASEBKGND, reinterpret_cast<WPARAM>(dc.GetSafeHdc()), 0);

		// Center icon in client rectangle
		int cxIcon = GetSystemMetrics(SM_CXICON);
		int cyIcon = GetSystemMetrics(SM_CYICON);
		CRect rect;
		GetClientRect(&rect);
		int x = (rect.Width() - cxIcon + 1) / 2;
		int y = (rect.Height() - cyIcon + 1) / 2;

		// Draw the icon
		dc.DrawIcon(x, y, m_hIcon);
	}
	else
	{
		CDialog::OnPaint();
	}

      //heres where i put the code to paint a PNG on runtime

       Rect rect(20,20,50,50);
       Graphics grpx(dc);
       Image * Img = Image::FromFile(L"DriveIcon.png",FALSE);
       grpx.DrawImage(Img,rect);
}

How can is use that to display the Image when i click a button or do some king of action?