@doctorcookies I think you probably already know how easy it can be to seek permission from (living) artists, and what a gratifying and mutually beneficial process this can be! You and NeGeRa seem to have done exactly this before you posted your Challenge #18 entry this week, which is great! 
Many artists are honored to be asked and willingly allow their work to be copied. Plus, there is then the mutual benefit of there now being two parties promoting the work.
However, I've also seen the opposite, where an artist has refused use for various reasons. I do this for some forms of my work (videos, work being used in classes or for other commercial purposes) under some circumstances; in some cases, I also require a licensing fee. In other cases, I allow use for free. But I always hope to be asked first.