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

\begin{tikzpicture}
% definitions
\tikzstyle{main}=[rectangle, minimum width=25mm, minimum height=30mm, 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=1cm]{
&  \node[main, minimum width=64mm, minimum height=30mm] (horizon) [] {\pgftext{\includegraphics[width=62mm]{csb_Time_horizon}}}; & \\
\node[main, minimum width=64mm, minimum height=36mm] (volume) [] {\pgftext{\includegraphics[width=62mm]{csb_Nourishment_Volume}}}; & & \node[main, minimum width=64mm, minimum height=25mm] (type) [] {\pgftext{\includegraphics[width=62mm]{csb_Nourishment_Type}}}; \\
  \node[main, minimum width=64mm, minimum height=56mm] (mklchange) [] {\pgftext{\includegraphics[width=62mm]{csb_MKL_change}}}; & & \node[main, minimum width=64mm, minimum height=76mm] (dfchange) [] {\pgftext{\includegraphics[width=62mm]{csb_Dune_foot_change}}};\\
  };
  % arrows
  \path (horizon) edge [connect] (mklchange)
        (horizon) edge [connect] (dfchange)
        (type) edge [connect] (volume)
        (type) edge [connect] (mklchange)
        (type) edge [connect] (dfchange)
        (volume) edge [connect] (mklchange)
        (volume) edge [connect] (dfchange);
  % 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}