We suppose to take, as example, a very good @Jake's solution: Drawing on an image with TikZ where I have put a generic image (I not remember where I have download it).
\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\node[anchor=south west,inner sep=0] (image) at (0,0) {\includegraphics[width=0.9\textwidth]{cat}};
\begin{scope}[x={(image.south east)},y={(image.north west)}]
\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}; }
\end{scope}
\end{tikzpicture}
\end{document}
The output is this:
I make my images in pdf using Adobe Illustrator. Is there a package that can generate this kind of callouts with comments that can be placed according to the coordinates of the grid. Probably a grid with squares, than the rectangle grid, could be more useful.
Here there is an example, where the line of the callout is fixed if I can to choose the coordinate of a point (x,y).
I don't remember where I downloaded the photo right now. Excuse me.



\usetikzlibrary{shapes.callouts}? (see my user profile ;-) – Sep 15 '19 at 20:44