\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{calc,decorations.markings}
\newcommand\TestF{\begin{frame}test\end{frame}}
\newcommand\vTestF{\TestF\TestF\TestF\TestF\TestF}
\newcommand\xxTestF{\vTestF\vTestF\vTestF\vTestF}
\def\factor{0.5}
\setbeamertemplate{background canvas}{%
\includegraphics[height=\paperheight,width=\paperwidth]{london}
}
\setbeamertemplate{background}{%
\setbeamertemplate{background}{%
\begin{tikzpicture}[remember picture,overlay,
decoration={
markings,
mark=at position \thepage/\inserttotalframenumber