Open Photoshop
Open the main image
Open the second image
Select the second image
Press Ctrl+A on Windows or Command+A on Mac
Press Ctrl+C to copy
Switch to the main image
Press Ctrl+V to paste
Use the Move Tool to position the picture
Press Ctrl+T to resize or rotate the picture
Press Enter to confirm the transform
Use a layer mask if you want to blend the images
Save the file in PSD format to keep layers
Export the final image in your preferred format