Kacper, the rotating pie is a cool idea. It is a bit clunky because 1). it is not quite turning from the center and 2) you only have the four stops. Try adding more stops on the dial to smooth it out (instead of the 4 stops try 16 or 32 then have your content changed based on 4 or 8 times the number).
To center the picture, draw a perfect square in your photo editing program (PowerPoint works fine.), please the picture in the center of the square using alignment tools. Group the picture and the square. Copy it. Past it as a picture. Then crop the square to a circle. Finally, turn the leftover parts of the circle to transparent by selecting a transparent color. Save your new picture and use it to replace the picture currently on dial. Let me know if these steps help.