Alright, first off, Portal 2 is awesome Ok now to the piece. I like the simplicity of your painting, it really gives the feel that she's entering a portal. ON dA. It seems that you worked really hard on Chell and the portal, but I think the other side of it could be worked a little bit more. I liked the lighting, but there is a small issue, the portal is already a weak source of light. In this case a faint blue glow should be visible on Chell, most likely on the boots as they have that reflecting characteristic.
I feel like telling you that I absolutely love your painting and drawing style. Awesome work as always