It looks like you've got natural light behind and to the right, probably windows. And incandescent light overhead and in front. Doesn't look like any flash. Right so far?

I'd actually shade the windows when you take pictures. I know you're supposed to use natural light when you can, but in your case the backlight is brighter than the overhead. There's an optical illusion that all comic artists exploit that brighter shapes pull forward, and shadows recede. With the back of the subject the brightest point, it flattens the image.

If you can position your subject so the natural light is the foreground, with the room lights behind, you'll get a much better shot. You want your brightest point in the photo to be toward the top/front of the subject. Then you'll want to auto-level (assuming your imaging program has that).

No matter what you use as your primary light, the brighter it is the better the photo will come out. You can fix a lot of sins in the Gimp, but the better the original the less work it will take.