Can i use images from the image site iStockPhoto for my animals?

You can, however that is a pay site, so you'd be paying for your images and would still need to credit them in full URL format to the photographer. 

We do have a Crediting Help page that includes a list of several free stock sites and how you'd credit, if that's at all useful:

https://horsephenomena.com/credithelp.php

So just saving the image wouldn't work? Ok, Thanks. :D