Last update of Sum and Blossom brought many animations and graphics changes. There is a lot of seamless background textures. I found a Gimp has a very useful layer transformation. Offset. This helped a lot.
MyPaint - I use it with my wacom tablet as a sketch book.
... but final work I have to do using the Inkscape - vector graphics editor. Below a screenshot with a final willow like world selector.
Bitmap fonts for openGL rendering are prepared with Hiero. And finally all graphic assets are packed in one bitmap with a TexturePacker - a part of libgdx framework.