Draw Circle About Figure

Question

I have a figure and would like to highlight a particular region of that figure with a circle or an ellipse as shown in figure:

It is important that the line of the circle or ellipse are thick. How do I control the location of the circle, to up, down, right or left?

Thanks a lot!

Answer 1

With TikZ:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{positioning,shapes}
\begin{document}

\begin{center}
\begin{tikzpicture}
\node(a){\includegraphics{circle.png}};
\node at(a.center)[draw, red,line width=3pt,ellipse, minimum width=118pt, minimum height=50pt,rotate=-28,yshift=-48pt]{};
\end{tikzpicture}   
\end{center}

\end{document} 

I used red so that you can see where my ellipse is with respect to the original ellipse. You can fine-tune the positioning via shifts, rotate, etc.

Answer 2

Here is a way by using \tikz for the ellipse, and then \stackinset to overlay it on the image. Arguments #2 and #4 to \stackinset are the offsets from the left, bottom of the image (horizontal offsets can be with respect to l, c, or r and vertical t, c, or b), while the 4 arguments to \solidcirc are the rotation angle, line thickness, and the x and y radii of the ellipse.

\documentclass{article}
\usepackage{stackengine,tikz,graphicx}
\newcommand\solidcirc[4][0]{\rotatebox{#1}{\tikz{\draw[line width=#2] (0,0) 
  arc [x radius=#3,y radius=#4,start angle=0,end angle=360];}}}
\begin{document}
\stackinset{l}{1in}{b}{.8in}{\textcolor{blue}{\solidcirc[-30]{3pt}{2.8}{1.0}}}
  {\includegraphics{example-image}}
\end{document}