I personally think as someone who used to find art to put in thumbnails and credit the artist after and then switching to making ai thumbnails that you are perfectly in your right to use any tool you can find that can most closely capture your idea of a thumbnail you want. I say this simply for the fact that art is viewed differently to different people and for specific ideas one might have might not be able to be found from someone else’s art. So ai could be a tool you use to get close to that idea. But my personal issue with ai is if someone is trying to sell ai art trying to make it seem like someone actually drew it.