Demo import problems are almost always server (PHP) limits being too low for the amount of content.
Ask your host to raise these, or set them in php.ini / .htaccess:
1. max_execution_time = 300 2. memory_limit = 256M 3. upload_max_filesize = 64M 4. post_max_size = 64M 5. max_input_vars = 3000
Then retry the import on a fresh install.
About images: the demo preview uses stock/placeholder photos that can't be redistributed, so they import as blank boxes or are skipped. This is expected — replace them with your own media. If existing images look wrong sizes after import, install "Regenerate Thumbnails" and run it once.