Source 4 isn't actually owned by the Deviant Art author, it's even stated in his description that he takes images from the net. The actual original can be found here: http://walkingwithdinos.wikia.com/wiki/Gorgosaurus .

It's a 3D render from "Walking With Dinosaurs" and since Photoshop Rules don't allow use of illustrations or 3D renders you'll have to replace it with a safe source.

5.6. 3D Renders or Illustrations as Source:
Using other people's 3D renders or illustrations (even with permission and no matter in which form they are offered) creates a false impression of your artwork. So 3D renders or illustrations can not be used as sources, unless they are entirely created by yourself. Only acceptable non-illustration brushes and clip art may be used, but you need to mention them as a source too. Don't use an image that has been altered by the source owner nor an entire work created by someone else.

You'll find some good dinosaurs on flickr like this one: https://www.flickr.com/photos/scottkinmartin/457428714

Use this link for more choices: https://www.flickr.com/search/?advanced=1&license=1%2C2%2C9%2C10&text=dinosaurs

I hope this helps.