IEBitmap := TIEBitmap.Create;
try
  // 1. Load the image first
  MemStream.Position := 0;
  IEBitmap.IO.LoadFromStream(MemStream);
  
  // 2. Resize (while still at full color depth)
  IEBitmap.Resample(24, 24, rfLanczos3);
  
  // 3. Configure for transparency AFTER loading and resizing
  IEBitmap.VclBitmap.PixelFormat := pf32bit;
  IEBitmap.VclBitmap.AlphaFormat := afDefined;
  
  // 4. Add to ImageList
  ImageIndex := FImageList.Add(IEBitmap.VclBitmap, nil);
finally
  IEBitmap.Free;
end;

IEBitmap := TIEBitmap.Create;
try
  // 1. Load the image first
  MemStream.Position := 0;
  IEBitmap.IO.LoadFromStream(MemStream);
  
  // 2. Resize (while still at full color depth)
  IEBitmap.Resample(24, 24, rfLanczos3);
  
  // 3. Add to ImageList
  ImageIndex := IEBitmap.AddToImageList( FImageList );
finally
  IEBitmap.Free;
end;