I copied and modified the code given on http://www.texample.net/tikz/examples/swan-wave-model/
\begin{figure}[H]
\begin{tikzpicture}[scale=.5,every node/.style={minimum size=1cm},on grid]
\begin{scope}[
yshift=-83,every node/.append style={
yslant=0.5,xslant=-1},yslant=0.5,xslant=-1
]
% opacity to prevent graphical interference
\fill[white,fill opacity=0.9] (0,0) rectangle (6,6);
\draw[step=5mm, black] (0,0) grid (6,6); %defining grids
\draw[step=1mm, red!50,thin] (0.5,0.5) grid (2,2); %Nested Grid
\draw[black,very thick] (0,0) rectangle (6,6);%marking borders
\fill[red] (1,1) rectangle (1.5,1.5);
%\includegraphics[scale=1,opacity=0.8]{Pics/face_patch}
%Idem as above, for the n-th grid:
\end{scope}
\begin{scope}[
yshift=90,every node/.append style={
yslant=0.5,xslant=-1},yslant=0.5,xslant=-1
]
\fill[white,fill opacity=.9] (0,0) rectangle (5,5);
\draw[step=10mm, black] (1,1) grid (4,4);
\draw[black,very thick] (1,1) rectangle (4,4);
\draw[red,dashed] (0,0) rectangle (5,5);
\end{scope}
\draw[-latex,thick](5.8,-.3)node[right]{$\mathsf{Skup\ svih\ piksela\ u\ } \boldsymbol{X}$}
to[out=180,in=90] (3.9,-1);
\draw[-latex,thick,text=black](5.9,5)node[right]{$\mathsf{Skup\ svih\ piksela\ u\ } \boldsymbol{x}_i$}
to[out=180,in=90] (3.6,5);
\draw[-latex,dashed,text=black](5.7,3)node[right]{$x_i$}
to[out=180,in=90] (0,5.7);
\draw[-latex,thick,red,text=black](-3.3,-3.2)node[right]{$x_i$}
to[out=180,in=90] (0,-1.4);
\draw[-latex,dashed,red,text=black](4.3,-1.9)node[right]{$\mathsf{Skup\ svih\ piksela\ u\ } \boldsymbol{x}_i$}
to[out=180,in=90] (1,-1.3);
\end{tikzpicture}
\end{figure}
What i want is to insert a background picture with some opacity (so that the grid is visible as well as the underlying colors and grids/borders), so that the picture is a background of the first grid (so the image is slanted so to match the grid)
How can i do that? Doing a pure /includegraphics didn't work at all (it didn't slate the image as well as the image size didn't match the grid size)