So what is the difference between a painting and an illustration?

From observation, an illustration is about great looking line work. It is the first thing you will notice.

Meanwhile, paintings are about edge designs and gradations.

Just a quick sketch for practice. I used Blender to make the basketball goal and hoops because it is a lot faster to determine the perspective of objects in a 3D software. I then painted over the goal in Paint Tool Sai.

The character, the clouds, and the sky were originally painted with Paint Tool Sai.

You can do this piece using Gimp. It is just that Sai has better blending brushes.

Blending modes is actually Gimp's weakness. It is very limited. If you want to achieve the effects that could be done in other painting software you would have to work harder.