Here are a number of ways of positioning elements horizontally within a frame:

\documentclass{beamer}
\usepackage{graphicx}% http://ctan.org/pkg/graphicx
\newcommand{\theimage}{\includegraphics[width=15mm]{tiger}}% Shorthand
\begin{document}
\begin{frame}
\hspace*{\fill}\theimage\hspace*{\fill}
\hspace*{\fill}\hspace*{\fill}\theimage\hspace*{\fill}
\hspace*{4cm}\theimage
\hspace*{\fill}\theimage\hspace*{4cm}
\end{frame}
\end{document}
From top to bottom the displacement represents:
Horizontally centred - similar to
\centerline{\theimage}
or
\begin{center}
\theimage
\end{center}
Horizontally positioned at with a ratio of 2:1 whitespace on either side (stemming from two \fills on the left and one on the right of \theimage). Other combinations will, of course, imply other ratios;
- Shifted a fixed length (
4cm in the example) from the left margin;
- Shifted a fixed length (
4cm in the example) from the right margin.
The use of \fill allows for something that has infinite stretch, while the starred version of \hspace is sometimes preferred when setting space from the text block boundaries inward.
In a similar way, adding a length <len> on one side within a centred image (say, left), pushes it away from that side (to the right) by half that distance (or .5<len>), as in @IanThompson's answer.
\begin{frame} ... \end{frame}over\frame{...}(nothing directly to do with the question in hand, but a generalbeamercomment). – Joseph Wright Feb 12 '12 at 21:49