I just saw @attercap saying that one of his favorite ways to do character portraits was to visit HeroForge, craft a minifig and use the .png export functionality. An excellent idea!
And just moments before, I had discovered 108 Terrible Character Portraits: free from DriveThruRPG, from a project realized back in 2012. Humans, elves, dwarves, trolls, robots, zombies, and more. CC BY 3.0 Unported License!