How to move a WordPress installation to a subdirectory

Move all your WP installation to the subdirectory, Then uodate in your DB (wp_options by default) the siteurl and home with the new url. change https://example.com to https://example.com/blog You need also to replace all ocurrences of example.com to example.com/blog in your DB manually, with SQL or with a plugin such Better Search Replace

Upload an image at frontend > Create direction and/or rename filename possible?

What the similar posts (wich your found) are suggesting, is adding a filter in your functions.php wich will handle the filenames for you. All uploaded files will be filtered by this function before the upload will be handled by wp_handle_upload. Add the following lines of code in your functions.php: function wpse_82741_rename_uploaded_file( $image ) { global … Read more