Ok, I found how to make credible swamps :
- new layer (over land & grass)
- layer fx : bevel & emboss / pillow emboss (size 3, soften 3)
- paint with a spatter brush, water color, 100 % (small touches)
It's visually acceptable, and easy to reproduce.

But I'm not satisfied with the jungle. So, one more try...


I will post a WIP when I'd recover my personal uploading space password, lol !