I have been using https://guff.szub.net/2006/02/09/post-image/ and it seems to work in the way you require… that is, you can write a post and upload an image with the same name as the post slug. And then include a string in the loop which will pull either a thumbnail or the image into the post.
All good, except, it doesn’t seem to work fully. The first post in the loop appears correctly, each subsequent post displays no image, and when I check the source for the page, content is missing from the src & alt tags.
Am continuing to search, and will post if I find a solution. Anyone else got any ideas?