Your feelings on the matter are very understandable.
'If' it happens here on Newgrounds you can use the little [Flag]-button on the submissions in question. (You don't need to write an essay. Just a link to your art and something along the lines of "This is my work - used without permission..." will already be enough.)
Yatsufusa
Should this ever become an issue (or if it already 'has' become an issue) legally operating sites let you send reports or have a DMCA-link.
Eymellia (Updated )
people use my art on their thumbnail without letting me know
I don't even know them
I don't like that...