Try to upload your images in their original resolution, as the site comes with a zoom in feature already. All I can really say about this piece is that maybe you should work on the edges, make sure they're all perfect (no jaggies or rough edges) and add some more detail.
Good work. In addition to what geonjaha said, I think a single additional colour for shading the snowmen and the snow would make this piece 10 times better. Unless you want to try dithering.