I'm trying to reproduce the following picture with TikZ:

Which is basically a circle with a variable width outline (the red line in the pic) that looks like a brush stroke. Can someone kindly give me a pointer to where to start?
A "poor man" solution:
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\node(a)[circle,fill=blue,minimum size=3cm]{};
\draw[line width=1pt,red,line cap=rounded] (a.180)to[bend left=45](a.90);
\draw[line width=1pt,red] (a.170)to[bend left=45](a.80);
\draw[line width=1.5pt,red] (a.160)to[bend left=45](a.70);
\draw[line width=2pt,red] (a.150)to[bend left=45](a.60);
\draw[line width=2.5pt,red] (a.140)to[bend left=45](a.50);
\draw[line width=3pt,red] (a.130)to[bend left=45](a.40);
\draw[line width=3.5pt,red] (a.120)to[bend left=45](a.30);
\draw[line width=4pt,red] (a.110)to[bend left=45](a.20);
\draw[line width=4.5pt,red] (a.100)to[bend left=45](a.10);
\draw[line width=5pt,red,line cap=round] (a.90)to[bend left=45](a.0);
\end{tikzpicture}
\end{document}

You can use a foreach loop to make the code prettier and/or add more steps so to have a smoother width increase.
You still need to implement the brush effect.
metapostthantikz... – cfr Jul 31 '14 at 00:42