If when you start your work, you're altering anything that already exists, be it a google image reference photograph, a street fighter sprite, some sprite you found in a forum or even an old nes mario sprite, then what you're doing is ripping/editing.
If you don't mention that you've done this when you post your art for critique, then you're being deceptive.
If you're working on a drawn & scanned base (be it a color reduced base or a traced base) that is anyone's but yours, and you don't give credit, you're being deceptive. Even if it is yours and you've color reduced instead of straight pixelled, you better mention that as it WILL show up in your final product and trained eyes will catch it and inquire.
If at any time when you work on your piece you find yourself thinking 'hm, should I mention this? Bahh. they'll never find out' then chances are, you're doing something you shouldn't be.
It's interesting that any of these things have to be spelled out for people.