The edit I made was actually quite a bit of work, involving a lot of playing around and tweaking over the course of a few days.
here's a few things that I really changed in your piece;
AnatomyYou gotta know your enemy before you can beat it. By the looks of your cat so far, I think it would be a good idea to study what actually makes cats who they are. They have strong back legs to be able to run quickly to both escape and over take. In stark contrast, their front legs aren't so nimble and mostly help with balance and shock absorbency in the front to increase the efficiency of the push from its hind legs. This is all fine and dandy, but these legs wouldn't be able to do anything without the complex bone structure going on underneath them; a single back leg has four main bone structures that work as hinges to enable the leg to move in a way that's both fast and physically possible. The first two on top are bent this way >, and supply a good deal of the push to move the cat, and because of this they have the most muscle on the leg. On down the line you've got the third forth (the forth being the foot) ones which are bent this way <; they provide some shock absorbency, but don't do too much in the way of propulsion so less muscle mass is needed in these parts. The front legs work pretty much the same way. The first two bones are positioned this way <, but at less of an angle then the hind legs, and mostly help absorb the shock of landing on the ground. They are also used to pull the cat forwards, and so they (mostly the top one) do have some nice muscle on them, but again, not nearly as much as the back legs. The third and forth one (the foot again) is just used to absorb more shock.
Now why would we need all of those shock absorbers? the answer is the body. The part that holds all of the organs and junk. Obviously we'll need space to fit all of the intestines, the stomach, the lungs, heart, you get the point. A cat's body is actually all below its spine, so it sinks down almost halfway past the legs to fit everything it needs to function right. Unlike dogs, who have a pretty apparent decrease in size past the ribcage, the cat's body is pretty much the same size all the way from the front to the back.
Have some pictures to figure out what exactly I'm talking about, and get a good grip on the proportions;
http://courses.washington.edu/chordate/453photos/skeleton_photos/cat_skeleton.jpghttp://www.pictures-of-kittens-and-cats.com/images/cat-skeleton.jpghttp://ropanimalhealth.com/files/blank_cat_exteral_anatomy.jpgThe third dimensionThe only medium that I know of that really uses all three dimensions would be sculpting. Every other medium needs to produce the
illusion of depth to really get a piece to shine. What I see in your cat right now doesn't lead me to think that anything is behind or in front of anything.
Like any sculpture, there will anyways be a part of your creation that will block your view of other parts of it, so you must be very aware of what's in front of, or behind, what you're drawing. This will help a lot with providing a good illusion of depth because it'll be a little bit more like sculpting out your entire creation. How I applied this in my edit; I first stretched its tummy down a few pixels, and while doing that I knew that the legs shouldn't get any shorter, so I sculpted the legs out a bit by making an outline, where needed, with the color just darker than what its tummy was. This enabled the legs to look closer than the body, and kept them the same length (because the fur covers most of the outlines I'm talking about, don't hesitate to ask if you need one without the fur to get what I'm saying).
ShadingThere are usually two stages people go through when learning to shade. The first is pillow shading, and then shading towards the light source without much thought about form. Your pieces are more in the second stage from what I've observed in the pieces you've shown so far. Just like everything, this takes mostly practice and trial 'n error to get the right priorities and form. It took me quite a few tries to get the back to look good with the rest of the piece, so don't be afraid to scrap your shading and try again if it doesn't look quite right.
Well, that's all I've got. I hope I've helped in some ways, and if you have any questions on why I did certain things that I failed to mention, please ask.
Hmm I was writing this up when you posted your update, and it seems I've edited the wrong sprite. Oh well, I think my critique can apply to your humanoid kitty a bit too. Also, don't worry too much about restrictions like tile sizes and things like that. They won't help your art improve very much.
