well depending on what you have, you can either get the game as a Rom on your computer, then take a screenshot of a sprite then carefully remove everything that isn't part of the sprite pixel by pixel or you can get sprites off of website
half of which STILL have a colored background you have to carefully snip away at pixel by pixel
depending on the size, it can take 25 minutes to weeks...too ke two weeks to make .png versions of Red, Green abd Touko/Touya's back sprites...
a way to cheat it to remove the pixels that surround the character, then simply erase everything else