Here’s an easy way to find the exact centre of your image in Photoshop using the rulers and guides.
With your document open in Photoshop, make your rulers visible (Ctrl+R in windows, Cmd+R on the Mac). Make sure that Snap is turned on, you can check this by choosing View > Snap. Then click and drag a guide out from the left hand-side ruler towards the middle of your document. The guide should snap automatically when it reaches the vertical centre. Now click and drag on the ruler at the top of the document window and drag down towards the centre. Again the guide should snap when it reaches the horizontal centre. And there where they meet, you have the very centre of your document.