Adding custom image in title slide

You can add an image on the title slide in beamer using \titlegraphic{}. Its position is dependent on, i.e. defined by, the used theme.

\documentclass{beamer}
\usetheme{Warsaw}
\title{Fooing the Bar}
\author{A.~Thor}
\institute{MWE Lmt.}
\titlegraphic{\includegraphics[width=\textwidth,height=.5\textheight]{someimage}}
\begin{document}
\begin{frame}
   \titlepage
\end{frame}
\end{document}

Alternatively you can place an image inclusion macro in one of the other title page macros, e.g.\institute{...} is good for adding the institute name as well as the logo. You are allowed to use line breaks there.

 \institute{Foo Research Institution\\[\medskipamount]
      \includegraphics[width=\textwidth,height=.5\textheight]{imgfilename}%
 }

Finally, you also can place the logo on an absolute position of the titlepage using tikz ortextpos. Here an example using tikz:

\documentclass{beamer}
\usepackage{tikz}
\usetheme{Warsaw}
\title{Fooing the Bar}
\author{A.~Thor}
\institute{MWE Lmt.}
\titlegraphic{\vspace{8cm}}% to push the other text to the top
\begin{document}
\begin{frame}
   \tikz [remember picture,overlay]
    \node at
        ([yshift=3cm]current page.south) 
        %or: (current page.center)
        {\includegraphics[width=\textwidth,height=.5\textheight]{someimage}};
   \titlepage
\end{frame}
\end{document}