ImageEnView1.LayersMerge(0, ImageEnView1.LayersCurrent, True, True);

procedure LayersMergeAll(Crop: boolean);

procedure TForm1.ImageEnVect1LayerNotify(Sender: TObject; layer: Integer;
  event: TIELayerEvent);
{ ImageEnVect Layer Notify. }
var
  iLayerPosX: Integer;
  iLayerPosY: Integer;
  iLayer0Width: Integer;
  iLayer0Height: Integer;
  iLayerWidth: Integer;
  iLayerHeight: Integer;
begin
  { Constrain XPos to layer 0 }
  if (layer <> 0) and ((event = ielMoved) or (event = ielMoving)) then
  begin
    iLayerPosX := ImageEnVect1.Layers[layer].PosX;
    iLayer0Width := ImageEnVect1.Layers[0].Width;
    iLayerWidth := ImageEnVect1.Layers[layer].Width;
    if iLayerPosX > iLayer0Width - iLayerWidth then
      ImageEnVect1.Layers[layer].PosX := iLayer0Width - iLayerWidth
    else if iLayerPosX < 0 then
      ImageEnVect1.Layers[layer].PosX := 0;
  end;

  { Constrain YPos to layer 0 }
  if (layer <> 0) and ((event = ielMoved) or (event = ielMoving)) then
  begin
    iLayerPosY := ImageEnVect1.Layers[layer].PosY;
    iLayer0Height := ImageEnVect1.Layers[0].Height;
    iLayerHeight := ImageEnVect1.Layers[layer].Height;
    if iLayerPosY > iLayer0Height - iLayerHeight then
      ImageEnVect1.Layers[layer].PosY := iLayer0Height - iLayerHeight
    else if iLayerPosY < 0 then
      ImageEnVect1.Layers[layer].PosY := 0;
  end;
end;

ImageEnView.LayersMerge([0, 1, 2]);  // merge layers 0, 1 and 2

ImageEnView.LayersMerge([]);   // merge all layers

// merge selected layers
procedure Tfmain.Button3Click(Sender: TObject); 
begin
  if ImageEnMView1.MultiSelectedImagesCount > 1 then
  begin
    ImageEnMView1.MultiSelectSortList(); // here we need sorted items
    ImageEnView1.LayersMerge( ImageEnMView1.MultiSelectedImagesList );

    RefreshControls();
    RefreshLayerViewer();
  end;
end;

This is a snippet of how I load up the images:
On form activate I set these values:
 ImageEnView1.LayersRotationUseFilterOnPreview :=true;    // make it fast preview
  ImageEnView1.LayersRotationDelayFilterOnPreview := true;


  ImageEnView1.SetLayersGripStyle(clBlack, clLime, bsSolid, 5, iegsCircle);
  ImageEnView1.ForceALTkey := true; // aspect ratio
  ImageEnView1.Blank;
  ImageEnView1.LayersSync := false;
  ImageEnView1.MouseInteract := [miMoveLayers, miResizeLayers {, miRotateLayers}];

This is my function to build the layout:
 function TFRM_collage.BuildCollage(Sender: tObject;
  layers: integer;
  Image1, Image2, Image3, Image4: string;
  shadow: boolean): integer;
var
  localconfigName: string;
  loop: integer;
  ThisIni: TIniFile;
  background: string;
  top: integer;
  left: integer;
  maxsize: integer;
  rotate: single;
  shadowradius: integer;
  shadowoffsetx: integer;
  shadowoffsety: integer;
  shadowintensity: integer;
  Ratio: double;
  schematic: string;
begin
  SaveMove1 := 1; // set them all to their default values.
  SaveMove2 := 2;
  SaveMove3 := 3;
  SaveMove4 := 4;
  ImageEnView1.LayersRotationFilter := ierBicubic;
  ImageEnView1.LayersRotationAntialias := true;

  result := 0;
  imageEnView1.Clear; // empty it all out
  LocalConfigName := Form1.DataBasePath + '\layout' + inttostr(layers) + '.CLD';
  if not fileexists(LocalConfigName) then
  begin
    result := loop;
    MessageDlg('The Required Layout File Could Not Be Found:' + #13 + #10 + LocalConfigName, mtError, [mbOK], 0);
    BTN_cancelClick(self);
    exit;
  end;
  ThisIni := TIniFile.Create(LocalConfigName);
  background := ThisIni.ReadString('fullpage', 'name', '');
  if not fileexists(background) then
  begin
    result := -1;
    MessageDlg('The Required Background File Could Not Be Found:' + #13 + #10 + background, mtError, [mbOK], 0);
    BTN_cancelClick(self);
    exit;
  end;

// put the bckground image
  imageEnView1.AutoShrink := true;

  imageEnView1.IO.LoadFromFile(background);
  imageEnView1.AutoShrink := true;
  imageEnView1.layers[0].locked := true;
  application.processmessages;

  BackgroundHeight := imageEnView1.layers[0].height;
  BackgroundWidth := imageEnView1.layers[0].width;

  for loop := 1 to layers do
  begin

    application.processmessages;

    top := ThisIni.ReadInteger('image' + inttostr(loop), 'top', 0);
    left := ThisIni.ReadInteger('image' + inttostr(loop), 'left', 0);
    maxsize := ThisIni.ReadInteger('image' + inttostr(loop), 'maxsize', 0);
    rotate := ThisIni.Readfloat('image' + inttostr(loop), 'rotate', 0);
    shadowradius := ThisIni.ReadInteger('image' + inttostr(loop), 'shadowradius', 0);
    shadowoffsetx := ThisIni.ReadInteger('image' + inttostr(loop), 'shadowoffsetx', 0);
    shadowoffsety := ThisIni.ReadInteger('image' + inttostr(loop), 'shadowoffsety', 0);
    shadowintensity := ThisIni.ReadInteger('image' + inttostr(loop), 'shadowintensity', 0);
    Layerrotateangle[loop] := rotate;

// Add the layer
    ImageEnView1.LayersAdd;
//    ImageEnView1.IO.Params.PSD_LoadLayers := true;

    if loop = 1 then
      imageEnView1.IO.LoadFromFile(Image1)
    else if loop = 2 then
      imageEnView1.IO.LoadFromFile(Image2)
    else if loop = 3 then
      imageEnView1.IO.LoadFromFile(Image3)
    else if loop = 4 then
      imageEnView1.IO.LoadFromFile(Image4);


    // Rotate has to be dome before placing
     if rotate <> 0 then
     begin
       imageEnView1.layers[loop].Rotate := rotate;
     end;


    if imageEnView1.layers[loop].Bitmap.Width > imageEnView1.layers[loop].Bitmap.height then // it is landscape
    begin
      Ratio := imageEnView1.layers[loop].Bitmap.height / imageEnView1.layers[loop].Bitmap.Width;
      imageEnView1.layers[loop].width := MaxSize;
      imageEnView1.layers[loop].height := round(Maxsize * Ratio);
    end
    else // no it is portrait
    begin
      Ratio := imageEnView1.layers[loop].Bitmap.Width / imageEnView1.layers[loop].Bitmap.height;
      imageEnView1.layers[loop].height := MaxSize;
      imageEnView1.layers[loop].width := round(Maxsize * Ratio);
    end;
    imageEnView1.Fit;

//    imageEnView1.layers[loop].Rotate := rotate;

    imageEnView1.layers[loop].PosX := left;
    imageEnView1.layers[loop].PosY := top;

//    imageEnView1.autoshrink := true;

    if Shadow then
      ImageEnView1.Proc.AddSoftShadow(shadowradius, shadowoffsetx, shadowoffsety, true, clblack, shadowintensity);

    application.ProcessMessages;
  end; //loop


  ThisIni.free;
  PNL_wait.Visible := false;
  screen.Cursor := crdefault;
  ImageEnView1.Update();
  application.processmessages;
end;

Then once the images have been move and rotated I do this:

  LocalConfigName := Form1.DataBasePath + '\layout' + inttostr(count) + '.CLD';
  if not fileexists(LocalConfigName) then
  begin
    MessageDlg('The Required Layout File Could Not Be Found:' + #13 + #10 + LocalConfigName, mtError, [mbOK], 0);
    BTN_cancelClick(self);
    exit;
  end;


  ImageEnView1.LayersRotationAntialias := true;
  ImageEnView1.LayersRotationFilter := ierBilinear;
  ImageenView1.LayersFixRotations();
  ImageenView1.Update();


  ThisIni := TIniFile.Create(LocalConfigName);

  for loop := 1 to count do
  begin

    ThisIni.writeinteger('image' + inttostr(loop), 'top', imageEnView1.layers[loop].PosY);
    ThisIni.writeinteger('image' + inttostr(loop), 'left', imageEnView1.layers[loop].PosX);
    ThisIni.writeinteger('image' + inttostr(loop), 'maxsize', imageEnView1.layers[loop].width);

    if Layerrotated[loop] then
    begin
      ThisIni.writefloat('image' + inttostr(loop), 'rotate', Layerrotateangle[loop]);
    end; //Layerrotated[loop]

  end; // loop
  application.ProcessMessages;
  ThisIni.free;

  // 20140806 added to merge all layers before finishing
  imageEnView1.layersmergeAll;



  ImageEnview1.IO.SaveToFileJpeg('c:\instevnt\collage\Collage' + inttostr(count) + '.jpg');
.....