Here you have two options, second is what Jake suggested and first was taken from
How to insert a background image in a beamer frame?
\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\usepackage{graphicx}
\begin{document}
{
\usebackgroundtemplate{%
\begin{tikzpicture}
\node[draw,very thick,minimum width=\paperwidth-\pgflinewidth,minimum height=\paperheight-\pgflinewidth] [anchor=north west] (mynode) {My node};
\draw (mynode.north west)--(mynode.south east);
\draw (mynode.south west)--(mynode.north east);
\end{tikzpicture}
}
\begin{frame}{Frame with background}
\begin{itemize}
\item 1
\item 2
\item 3
\end{itemize}
\end{frame}
}
\begin{frame}{Frame with overlay picture}
\begin{tikzpicture}[overlay, remember picture]
\node[draw,very thick,minimum width=\paperwidth-\pgflinewidth,minimum height=\paperheight-\pgflinewidth] at (current page.north west) [anchor=north west] (mynode) {My node};
\draw (mynode.north west)--(mynode.south east);
\draw (mynode.south west)--(mynode.north east);
\end{tikzpicture}
\begin{itemize}
\item 1
\item 2
\item 3
\end{itemize}
\end{frame}
\end{document}

EDIT
From beamer user guide 8.2 Frame and Margin Sizes
Aside from using these options, you should refrain from changing the “paper size.” However, you can change
the size of the left and right margins, which default to 1cm. To change them, you should use the following
command:
\setbeamersize{⟨options⟩}
The following ⟨options⟩ can be given:
- text margin left=⟨TEX dimension⟩ sets a new left margin. This
excludes the left sidebar. Thus, it is the distance between the right
edge of the left sidebar and the left edge of the text.
The problem is that this command is only valid in the preamble, so it changes all presentation margins. Here you have a new example:
\documentclass{beamer}
\usepackage{tikz}
\setbeamersize{text margin left=0pt}
\begin{document}
\begin{frame}
\begin{tikzpicture}
\node[minimum width=\paperwidth, minimum height=\paperheight, anchor=north west] (a) {};
\draw [very thick] (a.north west) -- (a.south east);
\draw [very thick] (a.north east) -- (a.south west);
\end{tikzpicture}
\end{frame}
\end{document}
and its result

\node at (current page.north west) [anchor=north west] {...};, there shouldn't be a gap. – Jake Jan 17 '12 at 08:50current pagenode then you have to useoverlay. – Andrew Stacey Jan 17 '12 at 09:22overlayfor histikzpicture. – Jake Jan 17 '12 at 09:36overlayandremember picturehave to be specified to use thecurrent pagenode. – Andrew Stacey Jan 17 '12 at 09:57handoutversion? Are you using anypgfpagesstuff when generating them? I find that messes upremember picturestuff, but there's a work-around. The alternative option is to have a frame style with no margins, in which case your question is "How do I define a beamer frame style with no margins and use it only on specific frames". Either way, a MWE would help considerably ... – Andrew Stacey Jan 17 '12 at 09:59remember picture,overlaysome of the others to this question might be of use: http://tex.stackexchange.com/questions/3915/image-on-full-slide-in-beamer-package – Andrew Stacey Jan 17 '12 at 10:17\begin{frame}[plain] ...\end{frame}– percusse Jan 17 '12 at 12:25sidebar? Eye-balling the shift amount, it really looks like there is an invisible sidebar sitting on the left. – percusse Jan 17 '12 at 19:13plain frameneeds hacked over. – benjamin Oct 19 '16 at 09:04