CButton *btnsample = (CButton *)GetDlgItem(IDC_BUTTON1);
btnsample->ModifyStyle(0,BS_ICON,SWP_FRAMECHANGED); //change the style of CButton
HICON hIcon = ::LoadIcon(AfxGetInstanceHandle(),MAKEINTRESOURCE(IDI_ICON)); //load an Icon assuming IDI_ICON is ID of ICON
btnsample->SetIcon(hIcon);