You open two pictures in photoshop, and then

You can use the move tool to drag one onto another, and dragging one will create a new layer. You can change the transparency so that the two images overlap, or you can use free transform to change the moving image and place it where you want.

In this way, if you want to change another picture, you must double-click the background and change it to layer 0 before you can change that picture.

The other is to create a new file, drag two pictures to the new file respectively, and two new layers will appear.

Then, use the selection tool (rectangle marquee \ circle marquee \ magic wand \ lasso \ pen) to select the area you want, and then select Copy in editing, then open the diagram or file you want to paste, and then click Edit and select Paste.