C / C++ / MFC

HANDLE handle = BeginUpdateResource(exe_file, FALSE);

   char *buffer;    // buffer to store raw icon data
   long buffersize; // length of buffer
   int hFile;       // file handle
       
   hFile = open(ico_file, O_RDONLY | O_BINARY);
   if (hFile == -1)
      return; // if file doesn't exist, can't be opened etc. 
       
   // calculate buffer length and load file into buffer
   buffersize = filelength(hFile);
   buffer = (char *)malloc(buffersize);
   read(hFile, buffer, buffersize);
   close(hFile);

   int icon_id=1;


	 UpdateResource(
	  handle,  // Handle to executable
	  RT_ICON, // Resource type - icon
	  MAKEINTRESOURCE(icon_id), // Make the id 1
	  
	  MAKELANGID(LANG_NEUTRAL, SUBLANG_NEUTRAL),

	  (buffer+22), 
	  // skip the first 22 bytes because this is the 
	  // icon header&directory entry (if the file 
	  // contains multiple images the directory entries
	  // will be larger than 22 bytes
	  buffersize-22  // length of buffer
	 );

  

	/////////////////////
   // Again, we use this structure for educational purposes.
   // The icon header and directory entries can be read from 
   // the file.
   GROUPICON grData;

   icon_id=1;

   // This is the header
   grData.Reserved1 = 0;     // reserved, must be 0
   grData.ResourceType = 1;  // type is 1 for icons
   grData.ImageCount = 1;    // number of icons in structure (1)

   // This is the directory entry
   grData.Width = 32;        // icon width (32)
   grData.Height = 32;       // icon height (32)

   grData.Colors = 0;        // colors (256)
   grData.Reserved2 = 0;     // reserved, must be 0
   grData.Planes = 2;        // color planes
   grData.BitsPerPixel = 32; // bit depth
   
   grData.ImageSize = buffersize - 22; // size of image
   
   grData.ResourceID = icon_id;       // resource ID is 1

   UpdateResource(
      handle,
      RT_GROUP_ICON,
	  
	  MAKEINTRESOURCE(icon_id),
      // MAINICON contains information about the
      // application's displayed icon
      
	  MAKELANGID(LANG_NEUTRAL, SUBLANG_NEUTRAL),

      &grData,
      // Pointer to this structure
      sizeof(GROUPICON)
     );

   delete buffer; // free memory
         

   // Perform the update, don't discard changes
  // Write changes then close it. 
	if (!EndUpdateResource(handle, FALSE)) 
	{ 
	
		return ;
	}