I discovered a funny thing:
Indeed I changed the "Date taken" of one picture I wanted to be the first in a folder (to be visible as Icon).
I changed the "date taken" to a date earlier than every other picture in the folder.
I synced my iphone with itunes...the order didn't change on my iphone..
I waited approx 5 minutes with itunes running ( just running, nothing else)
I synced again......and yes the picture was now on the first place.....
Possible explanation: itunes copied the picture during the first sync into the "Cache", but there's a background process analysing the "date taken" of the picture and putting it in the correct order afterwards....5 minutes later. After 5 minutes itunes "reordered" the pictures and the "new" order is synced during the 2nd synchronisation.
I hoped you understand what I mean .... 😉
Kind regards
gerardlu