There was another topic about ads, and people were saying it wasn’t possible. Well, what if I make a file on my server called “ad.html” and all it had was an ad. Can I display that webpage inside of the game?
You could just set up a mySQL database with the URL of the ad images and pick a random one through PHP to download onto Unity and put as a texture. 
Its simpler than it sounds. 8)
You can’t render HTML within Unity, but you can bring in images. Whether it is an HTML page or from a database, all you need to do is provide the URL to the image, and then bring in the image.