Alright, here goes.
*knucklepop*
First, I like your style of drawing. It's distinctive enough where I can look at a piece of artwork and knew that you drew it.
Unfortunately, that's probably the nicest thing that I have to say. You asked for critique, so I'm going to give you one. Keep in mind that this is just what I think of it, and you can take everything I tell you with a grain of salt.
Not to say that your artwork is bad, but you can definitely do a lot better.
Starting out with the fireflies picture:
I like the coloring for the sky and the texture you laid over top of it. But the image looks kind of flat, partially because of a lack of a horizon line and partially because of the fireflies. Objects farther in the background tend to 'fade' the farther back they are, even when giving off some light. So a firefly farther behind Kaz would look more blue tinted like the background. Right now, it looks like all of the fireflies could be on the same plane, but they're just different sizes.
You did some good calls with the lighting and darker coloring on Kaz, but his eyes shouldn't be perfectly white because of the lack of lighting. The moon shine on his head and shoulders looks good, and I like that there's some green light from the fireflies, but there are other light sources that seem to come from out of nowhere. It looks like you did a drawing of Kaz with neutral room lighting first, then added the background and light from the moon and fireflies as an afterthought. The composition also seems a little off to me, because there's a big empty patch of blue above his right (our left) shoulder. Even just adding some upper branches from a tree would add a lot to this image and help with the illusion of depth and perspective. The stars in the background also look a little sloppy. If some more time and care was taken with it, it would look better.
As a final note, I like how you included the craters on the moon, and I like how Kaz himself looks. Especially his nose. Seriously, that's a damn adorable nose.
Next, the portrait with the tan background:
Again, flatness. There's depth of color in his eyes and a shine on his glasses, but no other form of shading anywhere on the image. Also, his head and neck are huge compared to his shoulders. His ear looks too high up, his nose looks like it's not turned the way it should be, and his glasses (specifically the lens over his blue eye) look kind of wonky.
The lines look pretty good. Good line quality on a tablet is really hard to achieve unless you're using a cintiq or working on a huge canvas size.
Butterfly picture:
Kaz has got a bit of an Egyptian thing going on, meaning his head is turned to the side, but his body is mostly facing forward. This could be a legitimate pose, but the way his neck is drawn makes me uncertain about it. His waist also seems to be too small for his position, and his arms look like they're different lengths. Everything above from where his chest ends looks pretty good, though.
It looks like you used the burn tool to shade. Burning to shade can be very useful if done right, but there's a spot on his forearm where two strokes overlap and made an area darker than it's supposed to be. Try using a different layer over your flat colors to shade, and play with the opacity/layer mode of your shading color to achieve different shading effects.
Last note: The butterfly looks really cool, and I dig the scars and burns, especially the ones over his chest. When you draw the scars, try to keep in mind that their shape will warp to the contours of his body.
Is his eye that's showing supposed to be blue, or was that something that happened in RP after this picture was drawn?
Finally, the sneer.
I actually like this one best. There's a lot of attitude here, and it portrays the character well. You have a nice mouth, and mouths can be difficult to draw. You also did a lot nicer work with the shading. The yellow-green glow around him helps pop him off the background, and it compliments the pink of his bandana. The only thing I have to say that's negative about this is that it looks like it's kind of fuzzy/blurry. That could just be the image compression...
Again, there's the eye question that I had for the one before this.
Some last thoughts to wrap up. You can definitely draw, and you definitely know how things like lighting, anatomy, and color theory work. But it also feels like you don't want to fail with expanding past what you already know you can do well, so you stick to your comfort zone. If you really want to get better as an artist, sketch like crazy. Do studies. Work from references for poses, color, and lighting. They can be photographs or actual people. The internet is also your greatest resource. There's all kinds of information readily available for perspective, backgrounds, and anatomy. Hell, there's even a slew of Photoshop tutorials out there about how to do all sorts of neat tricks. Deviantart and Google are great resource centers.
The main thing to keep in mind is that studies and sketches don't need to be finished. They don't need to be perfect the first time around, or even perfect at all. And if you don't like them, you don't have to show them to anybody.
I can't wait to see more stuff out of you.
