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.current page
node then you have to useoverlay
.overlay
for histikzpicture
.overlay
andremember picture
have to be specified to use thecurrent page
node.