Find wanted image as big as possible to get the best resolution.
Drag the image on to the desktop / documents file.
Open or select the wanted Photoshop file.
Go to 'File', 'Place', and find the image you dragged onto the desktop or saved into the documents file, select it and click 'place'.
The Image should appear on the page, you can change the size of the image until it is how you want it and then press 'Enter'. !WARNING!: once you press enter you cannot change the size again, to do this you will have to redo this tutorial and change the size that way!
This is a good start and you have the basics here. However, this does not have a huge amount of detail and you will need to explain your answer more.