I really love this image - I'm a big mingus fan, and think that this is perfect for him. But as it is a WIP, I feel I can tell you what I think could be improved upon; I hope that's okay. The (what looks like but isn't) marker lines on the bass detract from the form of the bass and are distracting. Also, Mingus' hand looks like a blob; it could use a little more definition.
you and your WIPs... i personally think this looks great as it is. the 'not so finished, and yet quite finished' look this has going is really blunt and brilliant in its presentation, imho. But, let the artist to his own devices, it'll be interesting seeing how this progresses