A photo study painted in GIMP. Tumblr Pic with some steps: http://twarda.tumblr.com/post/90372943129/a-quick-photo-study-painted-in-gimp-reference
Reference used:
http://thetamar.deviantart.com/art/Valencia-Biopark-157-357077042
Similar studies:
https://www.weasyl.com/submission/696591/lion-compilation
https://www.weasyl.com/submission/215015/bovidae-compilation