You can either manually insert the image tags – try adding one through the media library first to see the code it generates, then go into the Text Editor rather than the Visual Editor and change the URL to one of your images – or you can code a theme template that would match up the day and automatically show the images.
You’d create a child theme, copy your parent theme’s post.php file, and then create some logic – get the current post’s published or last updated date, then check the folder you manually upload to to see if there are any images whose filenames start with that date (making sure to format it in the same way your filenames are formatted). If so, do a foreach loop, and set up image tags. The biggest problem to doing it this way is you won’t have any meaningful alt text for either search engines or visitors who need it, such as those using screen readers. That’s another benefit of using the Media Library – makes it much simpler to include meaningful alt text.