Guys, somehow I missed this thread. There is a site called Canva, you can get thousands of images, logos, etc and design anything you want. The majority of the content is in the same playful style as Planet Zoo so perfect to blend in with the game. You can create an account for free and try it out, they do give you a selection of their icons at no cost. I have a pro account with them and it is only around 12-14 bucks a month so worth the investment. I have had the account for years and had no regrets. You can import other images from your own folders and personal art as well, so it does open many possibilities.
As far as any copyright issues, I have worked with a number of artists thru the years designing zoological exhibits and everything associated with them, it is highly unlikely that anyone will be bothered by virtual game players using images in their imaginary zoos that generate no profit in real life to them. The majority of these artists create these images as commission work for zoos/museums/exhibitions, they sell a particular logo's right to that institution for them to use it how they see fit. If anything you should worry about the zoos coming after you if you use their logos, and that is another very unlikely scenario, you are far more likely to get hit by a bus. In any case, as someone already mentioned, there were absolutely no issues with Planet Coaster, and I remember seeing plenty of Disney Art there, and no one in the world would come at you harder than Disney when it comes to using their art without permission. I guess that no one truly cares much about a virtual world that lives only inside our pc's and heads. Just have fun and enjoy the game.