It's easy to download photos from the internet, but in most cases, reusing them on your own site is copyright infringement. Photos that are classified as being in the "public domain" are the exception, so be careful when publishing photos or graphics on your site.
Also, with the ease of sharing photos on social media, it's good to know what you can and can't do with someone else's posted photos. Here's a short post from designtaxi.com that explains sharing rules on Instagram.