I have no photo but know what you are talking about. One description in Haythornwaite's book
(two different editions)

based on the French army
pattern)"rank distinction was carried on the kepi,
which had a lace knot on the crown continuing as vertical lines of lace at the top of the sides
of the cap: for subalterns these lines and knot were of a single strip of lace, for captains two
and field officers three." One photo I have of
Guynemer shows him in dk blue coat/jacket but with
a light blue or red kepi (while Fonck is in all
horizon blue). If there were definite regulations
for dress in the air service, many wore what they
preferred to wear (at least in British and French
services; the Russians also had quite a selection)
Some regulation dress (if it existed) might never
catch up with quickly promoted pilots assuming
they would ever wear it.