> There’s still one catch: When I change the caption key „location“ to „bottom“ together with „inbetween={\blank[-\strutdepth]}“ the picture is aligned with the black line (top of the first text line). 
> This would actually be the preferred solution also for „{left,high}“. Yet I couldn’t find a possibility to shift the picture vertically by some millimeters.
> Is there?

To whoever might be trying to get this to work, too:
\offset[y=2mm] *inside* \placeongrid will do this.

Best,
Florian.