Wherever you found the artworks, you didn't create them. You gave you the right to freely take it and convert it, and then asking people to credit YOU for making brushes from art that you didn't create. No matter what you think, as long as the original artworks are not from you and you hadn't credit them (give the link of the creator, aka Amy Hamilton), this is legally called theft.
So you've got a choice : getting informed about how the copyright works, then admit that you made a mistake, and then at least credit the person who spent hours to draw those pictures ; or keeping be narrow minded, and then your brushes will be report as an art theft, a copyright infringment. Which is illegal.
As simple as this.
I'm pretty sure you can understand that it's not pleasant to see people use our work without crediting, as you're asking the exact same thing. So try to think about what you do, and act as you want people act towards you.