Place image between nodes in tikz: How to achieve precise positioning [duplicate]

Question

I am trying to include a background image in a tikz/pgf drawing. I need to be able to draw nodes directly on certain features of the image, it thus needs to be precisely adjusted.

This would be easiest if I could somehow fix it to two nodes defining the corners of the image, like so:

\documentclass{article}
\usepackage{pgf,tikz}
\begin{document}
\begin{tikzpicture}
\node at (0,0) {a};
\includegraphics{something.png}
\node at (2,2) {b};
\end{tikzpicture}
\end{document}

Of course this does not do what I want, i.e. scaling the image so that it its top-right corner is at (2,2) and the bottom-left corner at (0,0):

Answer 1

The comment by @percusse led me to the right answer. It involves using the scope environment.

The following code is taken from @Caramdir's answer in Drawing on an image with TikZ:

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
    \node[anchor=south west,inner sep=0] (image) at (0,0) {\includegraphics[width=0.9\textwidth]{some_image.jpg}};
    \begin{scope}[x={(image.south east)},y={(image.north west)}]
        \draw[red,ultra thick,rounded corners] (0.62,0.65) rectangle (0.78,0.75);
    \end{scope}
\end{tikzpicture}
\end{document}

Answer 2

How about this?

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\node[inner sep=0pt] (img) at (0,0) {\includegraphics{example-image-a}};
\coordinate (a) at (img.south west);
\coordinate (b) at (img.north east);
\draw[blue,thick] (a)--(b);
\end{tikzpicture}
\end{document}