You're too kind to me, and you'll have to thank first Daelin and - Max - as I took great inspiration from Maffaton & Yphyrion CitiesBy aquarits
Anyway your map is great and still is a reference for me.
I'm not proficient with GIMP... but after some missed tests, I found a way and here is a test... I've mixed feelings about it (looks fine but really makes the higher buildings standing out, no?).