I have just over 1300 posts on a TV streaming wordpress site that needs thumbnails. There will only be 20 unique thumbnails (one per TV show). You will need to download a relevant image, then use that image for the posts that match the show. (The posts are organised - each wordpress category is a different show).
Uploading the image to the post is easy. You just browse your computer and click upload, then click publish.
It could also be done in PHPmyadmin if you know how.