\documentclass{standalone}

\usepackage{gnuplot-lua-tikz}
\usepackage{tikz}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{mathrsfs}
\usepackage{amsmath}
\usepackage[amssymb]{SIunits}

\begin{document}
\begin{tikzpicture}[scale=1]

    \node at (0.5,-6.5) {\includegraphics[height=3cm]{photos/curvature2-2.png}};

    \draw[stealth-stealth, red] (-0.1,-6.68) --node[below]{$\ell$} (0.99,-6.68);
    \draw[stealth-stealth, red] (-0.1,-6.68+0.8) --node[left]{$2\zeta_m$} (-0.1,-6.68-0.8);


    \draw[thick, green] (1.1,-5.35) ++(265:1.3) arc (265:200:1.3);
    \draw[-stealth, yellow!60!red] (1.1,-5.35) -- node[right]{$R$} (0.3,-6.4);

\end{tikzpicture}
\end{document}