Figure labels not showing up

Question

I have the following code:

\documentclass{article}
\usepackage{graphicx}
\begin{document}
Introduction

\begin{figure}[h!]
  \includegraphics[width = 0.5\textwidth]{example-image-a}
  \label{fig:a}
  \caption{Image a}
\end{figure}

\begin{figure}[h!]
  \includegraphics[width = 0.5\textwidth]{example-image-b}
  \label{fig:b}
   \caption{Image b}
\end{figure}

Comparison of Fig. \ref{fig:a} to Fig. \ref{fig:b}.

\end{document}

After running pdflatex on the document twice, the figure labels do not show up:

What is wrong?