I need to build a presenting system (using a Raspberry Pi) which needs to be able to do all this:
Show a blank screen when started.
Show selected pictures and maybe even slide from one to another (if selected another one manually!).
Soft fade-in and fade-out would be very nice.
The controlling should be able to be done from the command line since I want to send a signal from outside to the system with the information on what to present.
Hope, anyone can help.
OSMC is not really designed for this and implementation of your requirements are likely not found in neat tutorials anywhere. There should be much easier ways to do this on a standard Raspbian install and you would find better info on that on whatever forum provides support for that OS.
I suspect here Raspberry Pi OS - Raspberry Pi Forums
What you want is possible in kodi, but would require either a add-on (written in python), or control through the JSON RPC interface.
texturecache supports some simpler JSON RPC calls. E.g.
./texturecache.py play <filename>
But something like screenly may be a better tool for this job.