In a composing class I took in college a few years back they said that 3 notes was the important number for creative license, and I think he sited Star Wars themes and lots of other John Williams compositions. I kinda look at it as, when you write a novel, and you use someone else's idea for a character name, a behavior, a location, or such, of the whole book how much actually ends up yours and theirs? When we make games, scripts let us do some things, but they don't 'make' the battles, they just let them run.
Sprites I have no idea about, but there's a game using Chrono Trigger duplicate sprites being released for GBA/Nintendo DS sometime soon that has actual copies of the full animations, just with sprite overs. Only reason I say this is, you can blatantly tell Seiken Densetsu 3 / Chrono Trigger rips are a big part of the commercial game, but they are fully edited, recolored, and altered. Square/Enix closed down that Chrono Trigger 3D remake that was not being commercially done, but just for fun, so obviously they're serious about persuing legal action, yet this other game has a release date this fall, or early spring.
Here it is, just found it.
http://www.4colorrebellion.com/archives/2006/06/10/4cr-interview-studio-archcraft/
I've always wondered, frankenspriting, does that make it yours, since what's changing is each pixel with the new color, and each alteration makes it less like the original. I dunno though, just my two cents. But from a moral standpoint you should give credit where it's due, and if making money offer some sort of financial reimbursement.