I'm generally using something along these lines:
\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
%\setlength\PreviewBorder{2mm} % use to add a border around the image
\usepackage{tikz}
\begin{document}
\begin{preview}
\begin{tikzpicture}
\shade (0,0) circle(2); % background
\draw (0,0) circle(2); % rim
\draw (.75,1) circle(.5); % right eye
\fill (.66,.9) circle(.25); % right pupil
\draw (-.75,1) circle(.5); % left eye
\fill (-.66,.9) circle(.25);% left pupil
\fill (.2,0) circle (.1); % right nostril
\fill (-.2,0) circle (.1); % left nostril
\draw (-135:1) arc (-135:-45:1) -- cycle; % mouth
\end{tikzpicture}
\end{preview}
\end{document}
The pdf file generated contains the standalone TikZ picture. To convert it to any other file format, simply open it with Gimp.
