Should I include a reference for an image that is released into the public domain?
Should I still include a reference to indicate this image is not my
  own work


YES If the idea is not yours, you need to reference it.


  Who should be given credit


That is a harder question. Ideally, you should find the original author/source. Given a reasonable attempt to find the original author/source fails then reference the secondary/reproduction (i.e., wikipedia/wikimedia).