Manually centering an Image or Layer in Photoshop is almost impossible. Thats why Photoshop has some alignment options to do the job for us automatically. To do this just select an Image or Layer by pressing Ctrl(Win)/Command(Mac) + A. Now choose the Move Tool press V on key board. Now look at the tools option under Photoshop menu bar. On the third section of the option bar we have alignment options. Click on Vertically center and Horizontally center button and you have your Layer or Image in the exact center of the document or canvas.