Bem, eu encontrei uma solução usando TikZ dentro de um documento LaTex elaborado. O resultado não é exatamente o mesmo, mas acho ainda melhor:
Issoexigiuterumdocumentotexcomespaçosreservadosqueserãosubstituídospelosargumentosparaumscriptsh.
%file:add_legend.tex\documentclass{standalone}\usepackage{graphicx}\usepackage{tikz}%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%LaTeXOverlayGenerator-AnnotatedFiguresv0.0.1%Createdwith(omittedhttp)ff.cx/latex-overlay-generator/%Ifthisgeneratorsavesyoutime,considerdonating5,-EUR!:-)%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\annotatedFigureBoxCustom{bottom-left}{top-right}{label}{label-position}{box-color}{label-color}{border-color}{text-color}\newcommand*\annotatedFigureBoxCustom[8]{\draw[#5,thick,roundedcorners](#1)rectangle(#2);\nodeat(#4)[fill=#6,thick,shape=circle,draw=#7,innersep=4pt,font=\huge\sffamily,text=#8]{\textbf{#3}};}%\annotatedFigureBox{bottom-left}{top-right}{label}{label-position}\newcommand*\annotatedFigureBox[4]{\annotatedFigureBoxCustom{#1}{#2}{#3}{#4}{white}{white}{black}{black}}\newcommand*\annotatedFigureText[4]{\node[draw=none,anchor=southwest,text=#2,innersep=0,textwidth=#3\linewidth,font=\sffamily]at(#1){#4};}\newenvironment{annotatedFigure}[1]{\centering\begin{tikzpicture}\node[anchor=southwest,innersep=0](image)at(-0.75,-0.75){#1};\begin{scope}[x={(image.southeast)},y={(image.northwest)}]}{\end{scope}\end{tikzpicture}}%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\begin{document}\begin{annotatedFigure}{\includegraphics[width=1.0\linewidth]{_image_}}\annotatedFigureBox{0,0}{0.000,0.0}{_letter_}{0,0}%bl\end{annotatedFigure}\end{document}
Eoscriptsh:
#!/bin/sh#Callthisscriptwithatleast2parameters,forexample#shscriptname<image_file><letter_of_legend>catadd_legend.tex|sed"s/_image_/$1/g" | sed "s/_letter_/$2/g" | pdflatex
#rename output to match <letter_of_legend>_<image_file> format
mv texput.pdf $2_$1
#clean up
rm texput.*
exit 0
Finnaly, chamando:
$ > ./legend.sh online_gauss.pdf A
a saída desenhada em "A_online_gauss.pdf"!