Updated April 2019.
Finding free stock photos for your blog posts is probably tougher than writing the actual post. Here's a list of go-to resources for free stock photos for use on your site.
Burst: Provided by Shopify, these feel a bit more staid.
Styled Stock: More "feminine" images—meaning lots of white backgrounds, flatlay images, and bright colors.
Negative Space: Interesting range of imagery.