These are quite amazing. Only suggestion might be to add a transparent version with just the outer black removed, not any internal pixels that are supposed to be solid.
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I actually did it myself by color replacing the background with 0 alpha, but I noticed that some of the colors that are supposed to be the same like the black background actually are made up of slightly different rgb values. Both the background and sprites I believe. You can't even tell with the naked eye, but maybe you want to fix it.