\documentclass{standalone} 
\usepackage[english]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{newtx}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\node[above right] at (0,0) {\includegraphics[scale=0.5]{pam.pdf}};
\draw[step=5mm,draw=gray,dashed] (0,0) grid (11,7);
\foreach \i in {0,...,11} \node[below] at (\i, 0) {\i};
\foreach \i in {1,...,7} \node[left] at (0, \i) {\i};

\node[right] at (4,4) {$s_i=\frac{b_i-a_i}{\max(a_i,b_i)}$};
\node[right] at (4,2.2) {\begin{minipage}{6cm}
\small
For the $x_i$ point:
\begin{itemize}
\item $a_i$ -- average distance to all other points in $x_i$ cluster
\item $b_i$ -- minimum of average distance to points in any other 
  cluster
\end{itemize}
\end{minipage}};
\end{tikzpicture}
\end{document}