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Any Drawing software like Paint Tool SAI, Paint.net or even some browser programs like Pixlr should do the trick.
I assume you must be new to spriting, so you should get some character sprites in Spriters Resource and edit over them as practice first.
Of course, not the best option, but you can.
You need to rescale the sprite to the double of it's size to fit with Essentials scale, usually like this:
64x64 >>> 128x128
80x80 >>> 160x160
Don't forget to change to "Nearest Neighbor" when rescaling, or the sprite will become blurry