Super Mario Bros custom tileset

[Hoeloe]

Better, but not perfect. You still have some very jagged edges, for example. This comes from you not taking care when placing pixels. Because pixel art is in such a limited space, every single pixel is important. First, the outline does not need to be two pixels thick when it changes - it eats up the little space you have and makes things look messy (it also makes the sharp changes more pronounced, removing the illusion of a consistent line). Your lines are also inconsistent. A tip for keeping things smooth is to change values by increments. So, for example, your Goomba has a pixel pattern on the side of its head which goes 1-2-1-2-1. This meaning you use two pixels, then move out one and use one, then move out again and use two, etc. A better pattern might be 1-1-2-2-1. Notice how the number of pixels in the line changes as little as possible - this gives the illusion of a smooth curve.

Now, please don't take this personally, you're making progress. Just remember that you're unlikely to get things perfect on your first try, and the best thing you can do is listen to what people say and act on it - people here know what they're talking about.

Also, on an unrelated note, it helps to post pictures at their native resolution (i.e. 1 pixel on the sprite maps to 1 pixel on the screen). This is just so we can get an idea of how the pieces as a whole looks.