@EdS @herbert spawning external processes from appimages is a bit tricky. An appimage runs the application inside a special environment, that needs to be “reset” to its original state before invoking external programs.
An working and tested example is available in my GIMP appimages. They can invoke an external RAW editor (RT, DT, PF or nUFraw) that come either as a native executable or an additional appimage package.
I would be glad to assist the digikam developers in settings this up, even if my spare time is currently quite scattered and hence I might not be quick in reacting from time to time…
EDIT: one thing you could check already is if a native digital executable can correctly invoke the RT appimage. I am providing the latter personally, so I might be well placed to debug this part…