I feel that I'm guilty of something similar to this.
This is similar to what my friend warned me about with my graphic design stuff when I made a tumblr. The stocks I use are from fan art. I take them and I crop them, and tear them apart. I'd imagine the original artist would either be flattered or very pissed off that I took their artwork and mutilated it.
Sometimes I wish I could draw the stocks myself, but I'm not that talented and I don't mean to take any credit for that.