Your practice engine idea is really smart, but it isn't at all immediately apparent upon visiting the site. At first, your website looks like any "portfolio site" (deviant art, dribble, etc). To make the practice engine the focus of the website, I would consider a new homepage design that focuses on the "learn to draw" aka "practice engine" aspect of the website, as opposed to a portfolio showcase. There are lots of portfolio sites, but very few websites that teach you how to draw on a structured learning path. It is a great idea, stick with it!