First of all, I want to apologize for my very ugly paintover. I hope it will still be clear enough to get across what I want. 
So, my feedback would be very similar to what @nkirill already mentionned. Since you mentionned you'd like to do something for a card game, I'd say that the first thing I'd do is to recrop the picture to focus more on the character. The pictures on the cards are small so I think it's really important to have your main subject take as much space as possible (even if that means getting rid of the cool parts behind her seat)
Next, I feel that beyond the anatomy, the pose itself is a little akward. To me, with the way she's sitting while thrusting her chest forward and gazing somewhere outside the picture feels more like she's posing for the camera like a model. So I would have her interact with the scene more to create a better sense of story. Luckily, you have this glowy diamond thing beside here so you can use that as a point of interaction. So can have her in a bit more natural pose and have her looking at it. That will also help keep the viewer inside the piece since right now, since she's looking to the far right, it kind of drags us away from the painting
And finally, as far as lightsources go, you have choices to make. You have 3. The diamond, her computer screen and the ambient light from the background. I think that either of the first two can make for a good main lightsource, you just need to decide which on you think will make the piece read the best at a small size. I personally went with the glowy diamond, since I think it also create a nice focal point. It's probably best to think about the lightsource at the beginning stages though, since it will directly influence your rendering.
And that's all that I can think of. I think the main concept is nice, you have some nice rendering and the character has some cool design elements. I hope my feedback helped you in some way and sorry again for the bad paintovers.
