If my humble self may divulge complimentary information that shall hopefully aid the OP in his quest for custom sprite domination:
Check out this thread
. Essentially, currently there're two 'standard' sprite tools, PIXI and GIEPY. However due to reasons listed in that thread GIEPY shall become the new standard tool. Therefore I suggest holding off on using PIXI and use GIEPY from the beginning.
Heed my words, I've solely used GIEPY in my hack and pretty much got every sprite from the sprite section I've used so far (over 100) to work.