I don't see any free or open source sprites. There is nothing on the website indicating that they are free or open source. Are you asking someone to credit the creator of sprites they paid money to use? That's absurd to me. It's like wanting this guy to credit Verizon or Time Warner for the bandwidth he paid for. If Oryx sprites were open source, or free for noncommercial use, I think it would be a legitimate claim. Otherwise, not so much, because it boils down to advertising. Crediting someone else is advertising someone else, and I believe giving someone actual money automatically means they don't need to also advertise for them.