What you don't see is the fact that in the original image I have the outlines and surfaces separated as individual layers so I can easily re-colour them to make variations. Equally I can hide some layers such as the leaves.
This way it is much easier to manipulate the images and to create a richer variety of objects. Layered images can be seen as "source" files for flat/"compiled" bitmaps. Great to know that people actually use this editing-friendly layered-images technique for producing game art.
Good news for Linux-users! Automatically built binaries of FreeOrion's latest SVN revision are available from psitronic.de/div/FO. They will allow you to play the game without having to build it, which isn't that easy on some distributions. Best thanks to kroddn! Windows builds are also available, but they are not as fresh... Sorry dear Win-users, but I see no other way for you, but to switch to Linux ASAP. ;) Anyways, here's a gameplay video while you download: