I started a course on Schoolism on Digital Painting (www.schoolism.com), and I can tell you I'm having such a good time learning! Here are some of the assignments I have made, I'm not done yet wit the course, so expect more to come.
Lesson 1 - Visualizing Through Darkness
Lesson 2 - Colorizing from Black & White
Lesson 3 - Textures Part 1
Lesson 4 - Textures Part 2
Lesson 5 - The Smudge Technique
Lesson 6 - Custom Brushes
Lesson 7 - Vector
Lesson 8 & 9 Final work: Abominable snowman
Bonus assignment!