Hi,
I have seen this question at least 10 time on Kodi’s forum and no dev seems to have give any answer.
I can’t imagine this is a very complex problem : generate thumbnail for photo collection not only when user is browsing his photos but before, on-demand or automaticaly.
I did find a script that does that but only for movies/music not for photos 
Any idea?
This is a kodi specific issue and would likely be best addressed in their forum, sometimes it takes a while to get a response though.
I did not checked myself, but the last guy that ask for this feature told that the first request for this feature was made in 2009 
Yes, but this is a Kodi feature that you are asking for. OSMC has no bearing. You should be asking them.