hehe, Paint.NET is virtually open-source too (student project)
anyway, no matter which of the two you use (or any other graphics program for that matter), there are just a few simple things that make life such easy especially when already having a form/shape/design to work/start with.
in gimp this is this selection wizard tool (don't ask me how it's called in english..zauberstab in german) which can be found on the toolbox window, 4th icon from the upper left.
anyway use this to get the forms copied out of a screenshot/image and resize/rescale or do perspective distortions (my english fails here a bit, sorry) with those copied design elements.
It's stunning how easy this works once you got acquainted to the program. Anyway use the toolbox, don't be overwhelmed by those tons of filters and options inside the program..you'll probably never need any of these.
I started learning gimp while creating my planet images and I learned how to visually enhance them, clean them from pixel errors and artefacts, add shadow and glow right and so on.
Took me 3 days straight to get to know everything. I did use only one tutorial to get textures on a planet sphere but with the rest I just played around with it and did some trial and error.
The learning curve is pretty high with those 2D programs.
The important thing is you use the
layers right, anything else comes with experience
Since we have our new botf2 site with those nice editing features, more than one can work on a particuliar image. In gimp case, just upload the xcf-file there and we all can edit and improve the picture.