How do I align an image to centre?

It is enough to use the center enviroment:

\begin{center}
    \includegraphics{yourimage}
\end{center}

The third way is to use the figure environment, which is the best way in my opinion, since it provides the right mark-up. (beamer centers the figures by default.)

\begin{figure}
   \includegraphics{<your image>}
\end{figure}

In this case on may also add a caption with \caption{<text>}

Here are two ways of achieving this:

\begin{center}
  \includegraphics{<your image>}
\end{center}

or

\hfill\includegraphics{<your image>}\hspace*{\fill}

The former may add some vertical whitespace, while the latter centers its contents on the line used.