If all you want to do is to add an image in the background for the title page, the method described in the linked answer is not the most appropriate one, since setting the title page template demands setting all the elements of the template and their attributes (which is part of your problem (see below)).
A more convenient approach is to locally (inside a group) set the background canvas template:
\documentclass{beamer}
\setbeamercolor{background canvas}{bg=black}
\setbeamercolor{normal text}{fg=white}
\setbeamercolor{title}{fg=white}
\title{The title}
\author{The Author}
\institute{The Institute}
\begin{document}
\begingroup
\setbeamertemplate{background canvas}{%
\includegraphics[width=\paperwidth,height=\paperheight]{mushrooms}}
\begin{frame}
\maketitle
\end{frame}
\endgroup
\begin{frame}
test
\end{frame}
\end{document}
If you, however, want to completely change the title page template by setting the template from scratch, then:
Your title is not centered because your current settings don't tell LaTeX that you want it centered; in fact, the minipage you are using for the title is way too big, as you can see by temporarily disabling the colors and framing it:
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{libertine} % or any other font package (or none)
\newcommand*\quotefont{\fontfamily{LinuxLibertineT-TLF}} % selects Libertine for quote font
\usepackage{tikz}
\usefonttheme[onlylarge]{structurebold}
\setbeamerfont*{frametitle}{size=\normalsize,series=\bfseries}
\setbeamertemplate{navigation symbols}{}
%\setbeamercolor{background canvas}{bg=black}
%\setbeamercolor{normal text}{fg=white}
\pgfdeclareimage[width=\paperwidth]{mybackground}{bigdata410}
\setbeamertemplate{title page}{
\begin{picture}(0,0)
\put(-30,-163){%
% \pgfuseimage{mybackground}
}
\put(0,-110.7){%
\centering
\frame{\begin{minipage}[b][45mm][t]{226mm}
\centering
\usebeamerfont{title}{\inserttitle\par}
\end{minipage}}
}
\end{picture}
}
\title{Lost in data}
\author[Author et al.\@]{Author}
\institute{University of }
\date{}
\begin{document}
\begin{frame}
\titlepage
\end{frame}
\end{document}
Change 226mm to \textwidth and use \centering inside the minipage:
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{libertine} % or any other font package (or none)
\newcommand*\quotefont{\fontfamily{LinuxLibertineT-TLF}} % selects Libertine for quote font
\usepackage{tikz}
\usefonttheme[onlylarge]{structurebold}
\setbeamerfont*{frametitle}{size=\normalsize,series=\bfseries}
\setbeamertemplate{navigation symbols}{}
%\setbeamercolor{background canvas}{bg=black}
%\setbeamercolor{normal text}{fg=white}
\pgfdeclareimage[width=\paperwidth]{mybackground}{bigdata410}
\setbeamertemplate{title page}{
\begin{picture}(0,0)
\put(-30,-163){%
% \pgfuseimage{mybackground}
}
\put(0,-110.7){%
\frame{\begin{minipage}[b][45mm][t]{\textwidth}
\centering
\usebeamerfont{title}{\inserttitle\par}
\end{minipage}}
}
\end{picture}
}
\title{Lost in data}
\author[Author et al.\@]{Author}
\institute{University of }
\date{}
\begin{document}
\begin{frame}
\titlepage
\end{frame}
\end{document}
Perhaps you also want to reduce the height?
There's no title nor institution since your settings for the title page template don't have the corresponding inserts; where do you want this information to appear? Use \insertauthor, \insertinstitution at the desired locations. For example:
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{libertine} % or any other font package (or none)
\newcommand*\quotefont{\fontfamily{LinuxLibertineT-TLF}} % selects Libertine for quote font
\usepackage{tikz}
\usefonttheme[onlylarge]{structurebold}
\setbeamerfont*{frametitle}{size=\normalsize,series=\bfseries}
\setbeamertemplate{navigation symbols}{}
\setbeamercolor{background canvas}{bg=black}
\setbeamercolor{normal text}{fg=white}
\pgfdeclareimage[width=\paperwidth]{mybackground}{bigdata410}
\setbeamertemplate{title page}{
\begin{picture}(0,0)
\put(-30,-163){%
\pgfuseimage{mybackground}
}
\put(0,-110.7){%
\frame{\begin{minipage}[b][45mm][t]{\textwidth}
\centering
\usebeamerfont{title}{\inserttitle\par}\vfill
\usebeamerfont{author}{\insertauthor\par}\vfill
\usebeamerfont{institute}{\insertinstitute\par}
\end{minipage}}
}
\end{picture}
}
\title{Lost in data}
\author[Author et al.\@]{Author}
\institute{University of }
\date{}
\begin{document}
\begin{frame}
\titlepage
\end{frame}
\end{document}
Once you are happy with the settings, delete (or comment out) the \frame command:
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{libertine} % or any other font package (or none)
\newcommand*\quotefont{\fontfamily{LinuxLibertineT-TLF}} % selects Libertine for quote font
\usepackage{tikz}
\usefonttheme[onlylarge]{structurebold}
\setbeamerfont*{frametitle}{size=\normalsize,series=\bfseries}
\setbeamertemplate{navigation symbols}{}
\setbeamercolor{background canvas}{bg=black}
\setbeamercolor{normal text}{fg=white}
\pgfdeclareimage[width=\paperwidth]{mybackground}{bigdata410}
\setbeamertemplate{title page}{
\begin{picture}(0,0)
\put(-30,-163){%
\pgfuseimage{mybackground}
}
\put(0,-110.7){%
\begin{minipage}[b][45mm][t]{\textwidth}
\centering
\usebeamerfont{title}{\inserttitle\par}\vfill
\usebeamerfont{author}{\insertauthor\par}\vfill
\usebeamerfont{institute}{\insertinstitute\par}
\end{minipage}%
}
\end{picture}
}
\title{Lost in data}
\author[Author et al.\@]{Author}
\institute{University of }
\date{}
\begin{document}
\begin{frame}
\titlepage
\end{frame}
\end{document}
There are several ways to remedy this warning; load a font containing the missing font size (lmodern, for example) or use \let\Tiny\tiny.
title pagetemplate; with your current settings the title won't be centered; furthermore, you don't include author nor institution on the template so obviously they don't appear. – Gonzalo Medina Aug 03 '14 at 00:44