Almost there

Indeed, the pen tool has helped you create clean lines here. The body's proportions and such are all good.

The foot to our left is a bit of a problem. it looks kinda like it's ona bit of a side view, but it's too close to being in line with the other foot, so one would think it should look like a front view instead. That brings me to my next suggestion, which is sort of the same thing.
I think you have a good grasp on how a human body should look (or at least a female body). Seeing your "Nekos + Yuri" picture, I just think you need to work on drawing breasts better; they look like they're on a weird angle in that picture. Anyhow, I think your next big step as an artist is to make your art look more like art. I used to have this problem, too, and I still do have this problem a little bit. I could draw a picture of a person, but I couldn't make it look like it had life, like they were really trying to do whatever they were doing in the picture. It looked more like they were forcing a pose. Anyhow, that's something you need to work on. In this case, what you would do is have that leg on the left end a bit higher. You should make it look like it's in the background a little bit if you want it to be on a little bit of a side view, it would make the picture look like it has more depth. It looks like you were trying to do that already, but you still made the leg go down too far.

But even after you get the hang of foreshortening (or whatever they call it), you need to make the characters look more natural/relaxed and also draw backgrounds. I think the Art Portal wants complete pictures, not just character designs and such. If you add real backgrounds to your pictures, they'll look more like art instead of just drawings. I mean, drawings are art, but art is more complicated than that.