Right now, Porky's head is a little too circular. He normally has a wider head, seen here. Not a huge problem, you could probably extend the face a pixel or two from each side and tweak the outlines a tad. Right now, I would take out the shading and focus on the lineart. It's not bad, but it looks a bit boring (mirrored, nearly circular, etc.).
I would recommend widening the head, making the bow a little sharper (right now, it's a little too round and gets lost in all of the other roundness of the sprite), and tweaking the mouth, because the curvature for a circle that small makes the top and bottom look sharp (try to get the shapes accurate, and the shapes can always be slightly smoothed out with AA.) I'm only looking at the first Porky sprite for the moment, BTW.
Made an edit to just the mouth and nose to show you what I mean. Right now, worry about the editing the lineart, though.
I would recommend widening the head, making the bow a little sharper (right now, it's a little too round and gets lost in all of the other roundness of the sprite), and tweaking the mouth, because the curvature for a circle that small makes the top and bottom look sharp (try to get the shapes accurate, and the shapes can always be slightly smoothed out with AA.) I'm only looking at the first Porky sprite for the moment, BTW.
Made an edit to just the mouth and nose to show you what I mean. Right now, worry about the editing the lineart, though.