Text on Subfloat images

Question

I looked at some posts about putting text on images but could not find an answer that helps me. I would be happy if someone could suggest a new way or simply a way to implement tikz or overpic better, even though \begin{tikzpicutre} or \begin{overpic} hasn't worked for me. Also when I try \put it places the text behind the images.

I am writing a scientific report and I need to number my images as A, B, C, D which are enclosed inside the same \begin{figure} \end{figure}. I assumed because I am using \subfloat tikz or overpic doesn't really work.

Is there a way for me to put text on the top left corner of each subfloat image and possibly choose the color, font and the background of the text?

\documentclass[12pt,a4paper]{report}
\usepackage[pdftex]{graphicx}
\usepackage{url} 
\usepackage[bookmarks, colorlinks=false, pdfborder={0 0 0}, pdftitle={<pdf title here>}, pdfauthor={<author's name here>}, pdfsubject={<subject here>}, pdfkeywords={<keywords here>}]{hyperref} 
\usepackage{mathptmx}
\usepackage{subfig} %for sub figures
\usepackage{float}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{amsmath} %use with \mathrm for non italic 
\usepackage{upgreek} % for non-italic greek letters
\usepackage{amssymb}
\usepackage{setspace}
\usepackage{indentfirst}
\usepackage[percentpdf]{overpic}
\usepackage[style=nature]{biblatex}
\addbibresource{main.bib}
\usepackage{hyperref}




\begin{document}

\begin{figure}[H]
    \centering
     \hfill
   \subfloat{\includegraphics[width=0.45\textwidth]{Images/img1.jpg}}
 \hfill
    \subfloat{\includegraphics[width=0.45\textwidth]{Images/img2.jpg}}
 
         \subfloat{\hbox{\hspace{0em}\includegraphics[width=0.49\textwidth]{Images/img3.png}}}
        \hfill
        \subfloat{\hbox{\hspace{0.5em}\includegraphics[width=0.49\textwidth]{Images/img4.png}}}
    \caption{ a) img1 b) img2 c) img3 d) img4}
    
\label{fig:imgs}
\end{figure}

\end{document}

Answer 1

This uses \llap, \makebox and \raisebox to overlap the text. The first step is to measure the width and height of the image using a savebox.

I used \subfloat for familiarity, but since you know the width, you could just as easily used a subfigure. The second image was overlaid with a complete subfigure caption.

\documentclass[12pt,a4paper]{report}
\usepackage{xcolor}
\usepackage[pdftex]{graphicx}
\usepackage{url} 
\usepackage[bookmarks, colorlinks=false, pdfborder={0 0 0}, pdftitle={<pdf title here>}, pdfauthor={<author's name here>}, pdfsubject={<subject here>}, pdfkeywords={<keywords here>}]{hyperref} 
\usepackage{mathptmx}
%\usepackage{subfig}% obsolete
\usepackage{subcaption}
\usepackage{float}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{amsmath} %use with \mathrm for non italic 
\usepackage{upgreek} % for non-italic greek letters
\usepackage{amssymb}
\usepackage{setspace}
\usepackage{indentfirst}
\usepackage[percentpdf]{overpic}
\usepackage[style=nature]{biblatex}
\addbibresource{main.bib}
\usepackage{hyperref}

\begin{document}

\begin{figure}[ht]% friends don't let friends use [H]
  \centering
  \sbox0{\includegraphics[width=0.45\textwidth]{example-image-a}}%{Images/img1.jpg}}
  \subfloat[]{\usebox0%
    \llap{\makebox[\dimexpr \wd0-2pt][l]{% left side
    \raisebox{\dimexpr \ht0-\ht\strutbox}[0pt][0pt]% top
      {\textcolor{red}{Text goes here}}}}}
  \hfil
  \sbox1{\includegraphics[width=0.45\textwidth]{example-image-b}}%{Images/img2.jpg}}
  \begin{subfigure}[b]{\wd1}
    \usebox1%
    \llap{\raisebox{\ht1}[0pt][0pt]{\begin{minipage}[t]{\dimexpr \wd1-4pt}
      \captionsetup{font={color=red}, skip=0pt, singlelinecheck=off}%
      \caption{A subfigure caption}
    \end{minipage}\hspace{2pt}}}
  \end{subfigure}
  
  \vskip\floatsep
  \subfloat[]{\includegraphics[width=0.49\textwidth]{example-image-c}}%{Images/img3.png}}}
  \hfil
  \subfloat[]{\includegraphics[width=0.49\textwidth]{example-image}}%{Images/img4.png}}}
  \caption{ a) img1 b) img2 c) img3 d) img4}% begins and ends with \par  
\label{fig:imgs}
\end{figure}

\end{document}