Caveat: Don't end a pycode environment with a comment, it will cause harm.
The caption package has the good intention of improving the appearance of figure captions. Unfortunately it changes the default style in a non-verbose way, which makes it dangerous to use in documents with existing formatting rules. Furthermore the subcaption package does not really tell that it loads the caption package behind the scenes.
Current answer: An axis environment is a special tikz node. It can be
given a name using the option [name=abc] and be referenced as such. It
can be positioned using the options [at=(...)] and [anchor=...]. It can
NOT be positioned with the positioning library for reasons unknown.
It is possible to add images to plots using
\node {\includegraphics[width=.5\linewidth]{image}}
or
\begin{axis}[width=.5\linewidth,
enlargelimits=false,
axis lines=none,
ticks=none,
scale only axis,
axis equal image]
\addplot graphics[xmin=0,ymin=0,xmax=320,ymax=200] {david};
\end{axis}
The latter is useful when more plots are added to the image. It is however painful that the image dimensions have to be specified for each and every image. The reason behind this is that the image is stretched to fit into the axis. The key [axis equal image] prevents this, but xmin,xmax,ymin,ymax still has to be specified manually, determining the image scaling.