REMOVING BACKGROUND FROM CHARACTER IMAGES

You know how Rpg Maker, VX has it so that if you import an image, there is a color remover to remove the background color so no blocks around your character or items etc. (only applies to PNGS however.)

However, in my case, my character set sheets apparently have like 4 shades of black or so, and have no idea how to get these out so the picture in the color in the background is removed while importing to VX.

If you could tell me how to do it, or if you could quickly do it for me, it's only 4 images, then it would be great. Thanks.