Unsplash

This site has a very good placement in media, but its images are dubious. None have model releases, a lot of the people are without heads and when you see a smile you can't be sure that smile won't sue you. Or the photographer. I've heard about one photographer who sued a user after giving away her photos for free. Ask yourself, why would photographers give away images? When everyone talks about privacy...
If a product is free than you must be the product.

A good site but i have requested my account be closed and all images removed, as i kept finding my images being sold on other sites. Not Unsplash fault as it is made clear, but too many scammers think it is ok to steal and sell images that don`t belong to them.

Unsplash offers many free stock images for download, and all you need to do is include some attribution to the image author on your site (put it into the "alt" values of the image).

They offer full sized image downloads (original size), so that is important because you can modify the image or resize it to fit any website, including the use of the image in a slider of a website because of its full size width.

I use Unsplash frequently for the free images because to have to purchase the images all the time will lead to a waste of money since what was thought to be a good image can turn out not as good when used on a website. The free image idea of Unsplash allows me to work images for websites much easier.

-- Premium Images --
It appears there are no premium images, all they offer are free images and they have a lot. I tried searching the site (after login to my free account) and there are no premium images, so it is not possible to compare price margins versus other premium sites like Shutterstock.

-- 4 Star Rating --
I gave 4 stars because of its downside.

--The Downside --
Their infinite scroll pages, which I dislike on any website.
I would rather have a long page that ends, and a "next page" button or link to go to the next page.
What all sites need are footers, which holds information I may need (ie: contact them), but cannot get to it because of infinite scroll. These infinite scroll designs are a nightmare to use.

Back when I worked in web design this was always my go to! Now, its great for stock photos as well. I really like the landscape vs portrait feature.