Hi Laurentino! You are doing pretty well here with your alien pic!
Is good that you flipped it, It helps lots! I have to say I took the liberty to paste your image in Photoshop and flipped it to see. that one you fixed already I don’t see it so bad, I do that all the time with my work and most of the time the eyes are un aligned and the heads kinda point to the left, I don’t know if this happened to you, But I’d suggest to keep working in ‘flipped’ until you feel you fixed the most of it, but keep flipping back to see it looks good both ways.
I was wondering if you started with a black background and then added lights, It looks like you did because your shadows tend to become grayish, which kills pastel colors like skin too easily, I suggest that first you boost the saturation of your whole image a bit, then grab an airbrush brush at, say… 30% opacity and add more color in the zones where light and shadow meet, lets say a good brown in the hue of orange for the skin, and so, don’t be afraid to try many hues of green for the alien skin, it could be more even more yellowish in the features of the face or at the ends of tentacles, who knows what color are they anyway? but, hey! lets also check the light source…
You used a light from the left, but also a back light from the right, which creates some sort of aura around the kid (he has light borders at booth sides) that is a good choice since it makes the kid stand up against the busy tentacle shapes, but you have to justify that light when you render the background so it’ll feel more natural, also, for this kind of image and it’s dark background you could use a colored backlight, it will be less strong than the main light source and also more easy to tell from each other, so it doesn’t appear that the kid has an outer glow.
As for the face there’s nothing better than googling for smiling kid pictures and grab some of this and some of that, your main problem is with the shadow under the cheeks, that you need to add so the face won’t look flat, also shape the chin (As you can see you have no shadow under it). Your firs post of the head (the darker one) Is actually real good, It even has greenish hues on the skin, that helps for natural look, and his expression is clearer in my opinion, again, the cheeks don’t have much volume, but you could borrow from the way you applied the shadow on that one and improve it with photo reference.
The backlight effect on the hair is very good, and that’s not easy to do. if you want to improve it, again your best bet is photo reference, and variation of hues (maybe a gentle amber shine on the right part).
I hope this helps a bit, see you around!!