Super Mario Bros custom tileset

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The head is too far back, move it a pixel or two to the right. The pose is really stiff with the straight arms and legs. Your shading doesn't add any volume, but follows the outline instead. You need to work on the colours - make your own choices instead of using presets, try colours with lower saturation (not 240 everywhere), shift the hue of darker shades (as in, shade darker blue or red with a bit more purple).

Those grey pixels you put everywhere around them will not work on varying backgrounds, they would stick out. Remove them and try a different sheet background colour, white usually isn't a good choice, some pale green should work better.