Finding suitable stock images for blog posts

We are migrating to a new web platform and have imported 1,850 blog posts from our old platform. We have all the blog post titles (and tags etc) down on a csv file, so this job would involve finding suitable free stock images (Pexels, Pixabay & Unsplash) for each blog post, then pasting the image link into the CSV file. Ideally, we would like each post to have a unique image.

A handful of the posts relate to specific dog breeds and sometimes the free stock sites don't have these images, but on the whole the posts are pretty mainstream, so images should be relatively easy to find.