As Ian said, credit whoever made the original pics use for the mishmash
Additional notes:
- if it's your art you drew yourself: do what you want
- if it's someone else's art (regardless of if you paid for it or not): ask for permission
- if you dont know who made the picture/dont have permission: don't use it
- I dont think crediting artbreeder is necessary. People don't credit Sai or Photoshop for the art they drew.