Scaling a virtual ruler (scale) to equally-sized pictures (for multiple pictures, 360° product photography)

I guess you want something like that or similar:

The following solution uses TikZ and TikZ-calc.

a. In the first step, I put the image in a node.

b. Then I add a simple coordinate system for that node.

c. With help of a scope-environment I can add any TikZ-stuff to the image inside the coordinate system.

d1. Hint: I use cmHgI.jpg from the startpost here; for users who do not have cmHgI.jpg
it will appear example-image.jpg

d2. The clue is to measure out the 'unitlength' from the ruler, shown in cmHgI.jpg and use it for all drawn TikZ-rulers.

€dit: I added a key \def\ShowHelps{1} % 1 'yes' 0 'no'

In a raw version this looks like:

Without the helping stuff:

\documentclass[a4paper, landscape]{article}
\usepackage[margin=2cm, showframe=false]{geometry} 
\usepackage{mwe} % Dummy images

\usepackage{tikz}
\usetikzlibrary{calc} 
\pgfkeys{/tikz/savevalue/.code 2 args={\global\edef#1{#2}}}

\def\ShowHelps{1} %    1 'yes'        0   'no'
\begin{document}
\centering\begin{tikzpicture}[font=\sffamily,
Help/.style={font=\Huge, red}
]
\node[anchor=south west,inner sep=0pt] (myimage) at (0,0) {    
\IfFileExists{cmHgI.jpg}
  {\includegraphics[scale=0.15]{cmHgI.jpg}}
    {\includegraphics[width=0.75\textwidth]{example-image.jpg}}
};
\begin{scope}[x={(myimage.south east)},y={(myimage.north west)}]
% Restrict the draw area
\clip (0,0) rectangle (1,1);
% Frame around image -  optional
\draw[] (0,0) rectangle (1,1); % optional
% CoSy inside node
\newcommand\ShowHelpCoSy{
  \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}; }
}
\ifnum\ShowHelps=1 \ShowHelpCoSy \fi%
\ifnum\ShowHelps=1\fill[Help] (0.5,0.75) circle(3pt) node[above]{(0.5,0.75)};\fi% <-- Position Test

% Now the stuff:
\path[] (0.705,0.4) coordinate(U1) -- (0.705,0.519) coordinate(U2);
\ifnum\ShowHelps=1 \draw[Help] (U1) circle(2pt) node[label=left:U1]{} -- (U2) circle(2pt) node[label=left:U2]{};\fi % show unitlength line

\path let \p1=($(U2)-(U1)$) in 
\pgfextra{  \pgfmathsetlengthmacro{\unitlength}{veclen(\x1,\y1)}   }
[savevalue={\u}{\unitlength}];
\ifnum\ShowHelps=1 \node[Help] at (0.725,0.25) {unitlength U1U2 = \u}; \fi%<--- show unitlength
\ifnum\ShowHelps=1 \draw[blue, transform canvas={xshift=2mm}] (U1) -- +(0,\u);\fi  % Test

% Small Ruler
\draw[very thick] (0.3,0.35) coordinate(A) -- +(0,3*\u);
\foreach \n in {0,1,...,3}{%%
\draw[very thick] ([yshift=\n*\u]A) -- +(-3mm,0) node[left]{\n} 
\ifnum\n=0 node[anchor=west, right=3mm]{cm}\fi;
}%%
\foreach \n in {0.1,0.2,...,3}{%% 
\draw[] ([yshift=\n*\u]A) -- +(-1.5mm,0);
}%%

% Image Ruler y
\draw[very thick] (0,0) coordinate(B) -- +(0,10*\u);
\foreach \n in {0,1,...,10}{%%
\draw[very thick] ([yshift=\n*\u]B) -- +(3mm,0) node[right]{
\ifnum\n=0 \else \ifnum\n=1 \n\,cm \else \n \fi\fi}; 
}%%
\foreach \n in {0.1,0.2,...,10}{%% 
\draw[] ([yshift=\n*\u]B) -- +(1.5mm,0);
}%%

% Image Ruler x
\draw[very thick] (0,1) coordinate(C) -- (15*\u,1);
\foreach \n in {0,1,...,15}{%%
\pgfmathsetmacro\Color{\n<9 ? "black" : "white"}
\draw[very thick, \Color] ([xshift=\n*\u]C) -- +(0,-3mm) node[below, text=\Color, inner sep=1pt]{
\ifnum\n=0 \else \ifnum\n=1 \n\,cm \else \n \fi\fi}; 
}%%
\foreach \n in {0.1,0.2,...,15}{%% 
\pgfmathsetmacro\Color{\n<9 ? "black" : "white"}
\draw[\Color] ([xshift=\n*\u]C) -- +(0,-1.5mm);
}%%
\end{scope}
\end{tikzpicture}
\end{document}