Tikz - highlight text in an image

Here is a recipe and an application. First the recipe, using this answer.

1. Add a grid on top.
2. Draw the contour you want to highlight.
3. Use the contour in an even odd rule fill (and drop the grid).

On purpose I used a document in which you may not want to use a horizontal box.

\documentclass[tikz,border=3.14mm]{standalone}
\begin{document}
\foreach \X in {0,1,2}
{\begin{tikzpicture}
    \node[anchor=south west,inner sep=0] (image) at (0,0) 
    {\includegraphics[width=0.9\textwidth]{Dunlap_broadside_copy_of_the_United_States_Declaration_of_Independence,_LOC.jpg}};
    % ^^^ https://upload.wikimedia.org/wikipedia/commons/4/4e/Dunlap_broadside_copy_of_the_United_States_Declaration_of_Independence%2C_LOC.jpg
    \begin{scope}[x={(image.south east)},y={(image.north west)}]
      \path[use as bounding box] (-0.1,-0.1) rectangle (1,1);
       % ^^^ only for animation
      \ifnum\X<2
        \draw[help lines,xstep=.1,ystep=.1] (0,0) grid (1,1);
        \foreach \x in {0,1,...,9} { \node [anchor=north] at (\x/10,0) {0.\x}; }
        \foreach \y in {0,1,...,9} { \node [anchor=east] at (0,\y/10) {0.\y}; }
      \fi   
      \ifnum\X=1    
        \draw (0.11,0.4) -- (0.37,0.404) --  (0.37,0.414) -- (0.11,0.41) -- cycle;
      \fi
      \ifnum\X=2
        \fill[even odd rule,opacity=0.4]
        (0.11,0.4) -- (0.37,0.404) --  (0.37,0.414) -- (0.11,0.41) -- cycle
        (0,0) rectangle (1,1);
      \fi       
    \end{scope} 
\end{tikzpicture}}
\end{document}

Of course, in the end you drop all the auxiliary stuff to arrive at

\documentclass[tikz,border=3.14mm]{standalone}
\begin{document}
\begin{tikzpicture}
    \node[anchor=south west,inner sep=0] (image) at (0,0) 
    {\includegraphics[width=0.9\textwidth]{Dunlap_broadside_copy_of_the_United_States_Declaration_of_Independence,_LOC.jpg}};
    % ^^^ https://upload.wikimedia.org/wikipedia/commons/4/4e/Dunlap_broadside_copy_of_the_United_States_Declaration_of_Independence%2C_LOC.jpg
    \begin{scope}[x={(image.south east)},y={(image.north west)}]
        \fill[even odd rule,opacity=0.4]
        (0.11,0.4) -- (0.37,0.404) --  (0.37,0.414) -- (0.11,0.41) -- cycle
        (0,0) rectangle (1,1);
    \end{scope} 
\end{tikzpicture}
\end{document}

Whether or not my choice of the highlighted text has anything to do with a wall at the border to Mexico, I do not want to elaborate on.

Depending on what format the images are, no tikz required:

\documentclass{article}
\usepackage{graphicx,stackengine,trimclip}
\begin{document}
\stackinset{l}{150pt}{b}{130pt}{%
  \clipbox{150pt 130pt 100pt 50pt}{\includegraphics{example-image.jpg}}%
}{%
  \includegraphics[decodearray={.3 .5 .3 .5 .3 .5}]{example-image.jpg}%
}
\end{document}

Depending on the color palette, the decodearray may need tweaking:

\documentclass{article}
\usepackage{graphicx,stackengine,trimclip}
\begin{document}
\stackinset{l}{20pt}{b}{40pt}{%
  \clipbox{20pt 40pt 15pt 15pt}{\includegraphics{example-grid-100x100bp.png}}%
}{%
  \includegraphics[decodearray={.3 .6 .3 .6 .3 .6}]{example-grid-100x100bp.jpg}%
}
\end{document}

decodearray will not work for cmyk color schemes, and only a limited number of image formats (jpeg being one).