There's nothing wrong to modifing a skin to your personal taste. But, if you want to share it with others, you have to ask the permission and give credit to the guy who created it in the first place. It's just a matter of common sense...
Best example here:
http://www.mp3car.com/vbulletin/rr-skins/68009-idrive-road-runner-originally-btbe.html