No one is going to get on you for plagiarizing someone's art, unless maybe you claim you did it and even then, we likely have no way of disproving it so really who cares. It's art that someone did, you are praising them by using it as a visual representation for your character. I do it for just about every RP I participate in because I'm a visual kind of person and to actually see something is just nice. The bottom line is like Plague said, the GM says have a picture, have a picture, not complain about it and be a baby and say how it's forced upon you. That's just childish. I do agree all in all that we should get passed this dumb shit and just prepare to RP some Mech whoop-assery.