Transparent foreground watermark

\documentclass{beamer}
\usepackage[printwatermark]{xwatermark}
\usepackage{xcolor}
\usepackage{graphicx}
\usepackage{tikz}
\usepackage{lipsum}

\newsavebox\mybox
\savebox\mybox{\tikz[color=red,opacity=0.3]\node{DRAFT};}
\newwatermark*[
  allpages,
  angle=45,
  scale=6,
  xpos=-20,
  ypos=15
]{\usebox\mybox}

\begin{document}

\begin{frame}
\lipsum[4]
\centering
\includegraphics[width=3cm]{example-image-a}
\end{frame}

\end{document}