\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{spy}
\begin{document}
\begin{tikzpicture}[spy using outlines={circle,yellow,magnification=5,size=.5cm, connect spies}]
\node {\pgfimage[height=2cm]{tux}};
\spy on (0,0.01) in node [left] at (1,.5);
\end{tikzpicture}
\begin{tikzpicture}[spy using outlines={circle,yellow,magnification=2,size=.5cm, connect spies}]
\node {$x^2+y^3+ \int\limits_0^a dx$};
\spy on (-.05,0.15) in node [left, fill = white, opacity=.8] at (0.27,0.25);
\end{tikzpicture}
\end{document}