Watermark in Tikz image

Question

I am finishing a math book. I want to know if it is possible to add a watermark in the lower right in most of my images created with Tikz, since those images are associated with a construction with GeoGebra (in addition to an augmented reality app) and that watermark helps to identify which images have a link (or augmented reality).

Some guide, I have no idea where to start.

I enclose a picture of what I want.

Answer 1

This is a quickly written answer (sorry, I am about to jump) that only deals with the main part of the question: adding the "watermarks". It does not discuss how to improve the layout/arrangements of the figure, which is certainly possible without minipages and, in particular, \\ and all these \vspace commands.

\documentclass[12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{tikz} %Encendido
\usepackage{tikzpeople}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}

\begin{document}

\textbf{Determinar cuando una gr\'afica corresponde a una funci\'on}

De las gr\'aficas que se presentan a continuaci\'on, se determina las relaciones
que corresponden y no a una funci\'on.\\

\noindent
\begin{minipage}{0.48\textwidth} 
    Corresponde a una funci\'on\\

    \begin{tikzpicture}[scale=0.6,local bounding box=TL]
    \draw [color=gray] (-3.5,-3.5) grid (3.5,3.5);
    \draw [latex-latex, very thick] (-4,0) -- (4,0) node [below] {$x$};
    \draw [latex-latex, very thick] (0,-4) -- (0,4) node [right] {$y$};

    \draw[latex-latex,domain=-2:2,variable=\x,color=blue, line width=1.2pt] plot ({\x},{\x * \x -1});
    \draw [line width=1.2pt,dashed,red] (-1,-3)-- (-1,3);

    \fill [fill=red] (-1,0) circle (3.5pt);
    \node[overlay,graduate,shield,minimum size=1cm,yshift=4mm] at (TL.south east){};
    \end{tikzpicture}

    \vspace{0.7cm}

    \rule{6cm}{0.1mm}\\

    \rule{6cm}{0.1mm}
\end{minipage} \hfill
\begin{minipage}{0.48\textwidth} 
    Corresponde a una funci\'on\\

    \begin{tikzpicture}[scale=0.6]
    \draw [color=gray] (-3.5,-3.5) grid (3.5,3.5);
    \draw [latex-latex, very thick] (-4,0) -- (4,0) node [below] {$x$};
    \draw [latex-latex, very thick] (0,-4) -- (0,4) node [right] {$y$};

    \draw[latex-latex,domain=-2.5:1.28,variable=\x,color=blue, line width=1.2pt] plot ({\x},{\x * \x * \x * \x +2 * \x * \x * \x -\x * \x - \x -1});
    \draw [line width=1.2pt,dashed,red] (-1,-3)-- (-1,3);

    \fill [fill=red] (-1,-2) circle (3.5pt);
    \end{tikzpicture}

    \vspace{0.7cm}

    \rule{6cm}{0.1mm}\\

    \rule{6cm}{0.1mm}
\end{minipage} 

\vspace{1cm}

\noindent
\begin{minipage}{0.48\textwidth} 
    No corresponde a una funci\'on\\

    \begin{tikzpicture}[scale=0.6,local bounding box=BL]
    \draw [color=gray] (-3.5,-3.5) grid (3.5,3.5);
    \draw [latex-latex, very thick] (-4,0) -- (4,0) node [below] {$x$};
    \draw [latex-latex, very thick] (0,-4) -- (0,4) node [right] {$y$};

    \draw [line width=1.5pt,color=blue] (2,-2) -- (-2,-2)-- (2,2) -- (-2,2);
    \draw [line width=1.2pt,dashed,red] (-1,-3)-- (-1,3);

    \fill [fill=red] (-1,2) circle (3.5pt);
    \fill [fill=red] (-1,-1) circle (3.5pt);
    \fill [fill=red] (-1,-2) circle (3.5pt);
    \node[overlay,graduate,shield,minimum size=1cm,yshift=4mm] at (BL.south east){};
    \end{tikzpicture}


    \vspace{0.7cm}

    \rule{6cm}{0.1mm}\\

    \rule{6cm}{0.1mm}
\end{minipage} \hfill
\begin{minipage}{0.48\textwidth} 
    No corresponde a una funci\'on\\

    \begin{tikzpicture}[scale=0.6]
    \draw [color=gray] (-0.5,-1.5) grid (6.5,5.5);
    \draw [latex-latex, very thick] (-1,0) -- (7,0) node [below] {$x$};
    \draw [latex-latex, very thick] (0,-2) -- (0,6) node [right] {$y$};
    \draw [line width=1.2pt,dashed,red] (2.,-1)-- (2.,5);
    \draw [line width=1.5pt,blue] (3,2) circle (2cm);

    \fill [fill=red] (2,0.28) circle (3.5pt);
    \fill [fill=red] (2,3.72) circle (3.5pt);
    \end{tikzpicture}

    \vspace{0.7cm}

    \rule{6cm}{0.1mm}\\

    \rule{6cm}{0.1mm}
\end{minipage} 

\end{document}