In Beamer, it is useful to position images absolutely. There are many ways to do
this, but one simple one-liner can be done with tikz.
\tikz[remember picture, overlay] \node[anchor=center] at (current page.center) {\includegraphics{foo}};
Compile twice to have the picture placed at exactly the center of the slide. The
anchors can be changed to move the picture around, and further adjustments can
be made by using the calc library.
% Preamble
\usepackage{tikz}
\usetikzlibrary{calc}
% ...
% Main document
\tikz[remember picture, overlay] \node[anchor=center] at ($(current page.center)-(1,0)$) {\includegraphics{foo}};
will place the image 1 cm to the left of the center.