How to add a photo to other one?

With background layer and Alpha channels that map image location position size and shape a script can populate your images over your background layer and position size and shape them to your mapping alpha channels.

Hi, It is also easier to drag and drop an image to another one if you go to windows>arrange>2up vertical.

You can also tear off the tabs to floating windows.

As JJ has told you, there are lots of ways.  

To open another image directly into the image you are working on: File > Place embedded.

Or open the second image so they are both open in photoshop. Go to the second image and drag from the layer panel to the original images tab (as below)

If you have an image in the clipboard, you can simply paste (as JJ said) with Cmd V which will create a new layer.

Edit Copy Paste,  Drag and  Drop, Menu File>Place.  Menu File>Scripts>Load file into stack, Frame tool etc.