Background image does not cover the entire page

A native beamer solution without additional packages:

\documentclass[14pt]{beamer} 
\usetheme{Warsaw}
\usecolortheme{wolverine}
\setbeamercovered{transparent}

\setbeamertemplate{navigation symbols}{}%remove navigation symbols

\begin{document}
{
\setbeamertemplate{headline}{\vskip\headheight}
\setbeamertemplate{footline}{}
\usebackgroundtemplate{\includegraphics[width=\paperwidth,height=\paperheight]{example-image}}%
\begin{frame}{}
\end{frame}
}


\section{Introduction}
\begin{frame}{a}
\end{frame}


\section{first part}
\begin{frame}{b}
\end{frame}


\end{document}

Set the image at page shipout as an overlay (using eso-pic):

\documentclass{beamer}

\usepackage{eso-pic}

\mode<presentation>
{
  \usetheme{Warsaw}
  \usecolortheme{wolverine}
  \setbeamercovered{transparent}
}
\setbeamertemplate{navigation symbols}{} % remove navigation symbols

\begin{document}

\begin{frame}{title page}
\AddToShipoutPictureFG*{
  \AtPageLowerLeft{%
    \includegraphics[width=\paperwidth,height=\paperheight]{example-image}%
  }
}
\end{frame}

\section{Introduction}
\begin{frame}{a}
\end{frame}

\section{first part}
\begin{frame}{b}
\end{frame}

\end{document}

\usebackgroundtemplate places the background image below the header. When you use \setbeamertemplate{headline}{}, you are only clearing the header contents, not deleting it, so what remains is the white background canvas.

One option is to define the background as an explicitly positioned tikzpicture. This will initially require two builds in order to place the image correctly, and the image will appear as the background for all frames. The following code shows the difference.

\documentclass{beamer} 
\usepackage{tikz}
    \usetikzlibrary{positioning}
\mode<presentation>
{
  \usetheme{Warsaw}
  \usecolortheme{wolverine}
  \setbeamercovered{transparent}
}
\setbeamertemplate{navigation symbols}{}%remove navigation symbols

\begin{document}
\setbeamertemplate{headline}{}
\setbeamertemplate{footline}{}

\usebackgroundtemplate{\includegraphics[width=\paperwidth,height=\paperheight]{example-image}}%

\begin{frame}{}
\end{frame}

\section{Introduction}
\begin{frame}{a}
\end{frame}

\usebackgroundtemplate{%
\begin{tikzpicture}[remember picture,overlay]
\node at (current page.center){\includegraphics[width=\paperwidth,height=\paperheight]{example-image}};
\end{tikzpicture}
}

\section{first part}
\begin{frame}{b}
\end{frame}

\end{document}