Draw symbols on top of external figure

Question

Say I have the following figure, which I want to add symbols on to:

The largest black square has dimensions -1 to 1 along the two edges. I want to place a triangle-symbol at the point (0.5, 0.5).

How is it possible to do this in either TikZ or PStricks? The first part is loading in the image and placing a symbol there, the other aspect is getting the location right.

Answer 1

\documentclass[tikz,border=3.14mm]{standalone}
\usetikzlibrary{plotmarks}
\begin{document}
\begin{tikzpicture}
\node at (0,0) {\includegraphics[width=2cm,height=2cm]{example-image}};
\node[blue] at (0.5,0.5) {\pgfuseplotmark{triangle}};
\end{tikzpicture}
\end{document}