\documentclass[border=2mm]{standalone}
\usepackage{graphicx}
\usepackage{epstopdf}
\usepackage{tikz}
\usetikzlibrary{arrows,chains,shapes,fit,shadows,calc,decorations,matrix}
\graphicspath{{../figures/theta311_radius16400/}}%  
\begin{document}

\begin{tikzpicture}
% definitions
\tikzstyle{main}=[rectangle, minimum width=22mm, minimum height=26mm, thick, draw =black!80, rounded corners]
%\tikzstyle{main}=[ellipse,fill=blue!20, draw=blue!20, drop shadow, thick, node distance = 40mm, minimum width=25mm]
\tikzstyle{connect}=[-latex, thick]
\tikzstyle{box}=[rectangle, draw=black!100, rounded corners]

  % nodes
  \matrix[matrix of nodes,row sep=-.5cm,column sep=.5cm]{
  \node[main, minimum height=31mm] (areacode) [] {\pgftext{\includegraphics[width=20mm]{debn_areacode}}}; & & & \\
  & \node[main, minimum height=21mm] (theta) [] {\pgftext{\includegraphics[width=20mm]{debn_theta}}}; & & \node[main, minimum height=21mm] (wavedir) [] {\pgftext{\includegraphics[width=20mm]{debn_wavedir}}}; \\
  \node[main, minimum height=21mm] (radius) [] {\pgftext{\includegraphics[width=20mm]{debn_radius}}}; & & \node[main, minimum height=21mm] (phi) [] {\pgftext{\includegraphics[width=20mm]{debn_phi}}}; & \\
  & \node[main, minimum height=21mm] (erosionfac) [] {\pgftext{\includegraphics[width=20mm]{debn_erosionfactor}}}; & & \\
  };
  % arrows
  \path (areacode) edge [connect] (radius)
        (areacode) edge [connect] (theta)
        (wavedir) edge [connect] (phi)
        (theta) edge [connect] (phi)
        (radius) edge [connect] (erosionfac)
        (phi) edge [connect] (erosionfac);
  % rectangles
%  \node[box, inner sep=2mm, fit= (radius) (erosionfac) (phi),label=below:Learn from XBeach simulations] {};
%  \node[box, inner sep=2mm, fit= (areacode) (radius) (theta),label=above:Learn from geometry data] {};
\end{tikzpicture}

\end{document}