These look cute but the dark background makes them unusable. One needs to remove it oneself. One could easily do so with the magic wand tool in any image editor, but you've incorporated the same color into some of the sprites (eyes and mouths for example), so merely deleting the color everywhere ruins many of them. As a mostly free asset, this probably isn't worth your time, but should you update these in the future, I would suggest removing the background and making it transparent. Nice work otherwise 👍