Olo, is it tiling knowledge you lack -OR- are you just looking for comment on the pixelling done, despite technical tiling techniques? I may be able to expand your understanding of square tiling systems.
As for the pixel art alone, IMO, there isn't nearly enough there to comment on. Whether or not they have sufficient contrast depends on other game art elements. At least post a test world, populated by tiles done thus far and we see if they're doing their job or not. Right now, you're asking us to judge the looks of a Miss America contestant by showing us only her left arm.