How do you want to merge them? Side by side or one within the other?
Essentially, all you need do is open both images in the Editor and click and drag one of the images from the photo bin to the other one that is open in the work space. You can then use the handles of each layer to resize and move the images about. If you need more information, please explain what you are trying to do.
In the following screenshot, I have created a new blank file and dragged the photo up to it and resized it.
If you're trying to merge two images of different sizes, the easiest way is to use 'Place Embedded' so the second image comes in as a Smart Object. That way you can just use the Transform tool (Ctrl/Cmd + T) to scale it down without losing quality.
@Sasha003, to be clear: in Photoshop Elements, there is no Place Embedded or Linked command. Those are only found in Photoshop CC (although Elements has a Place command). When drag and dropping an image from the Elements Photo Bin to the workspace, the newly created layer also comes in as a Smart Object and the layer handles are automatically available for adjusting size and position.
How do you want to merge them? Side by side or one within the other?
Essentially, all you need do is open both images in the Editor and click and drag one of the images from the photo bin to the other one that is open in the work space. You can then use the handles of each layer to resize and move the images about. If you need more information, please explain what you are trying to do.
In the following screenshot, I have created a new blank file and dragged the photo up to it and resized it.