Hello, @nedimzidane,
This is a known situation, not a bug. Images and hierarchy of categories can’t be imported, so you need to manually add images for demo categories, or delete them and add your own product categories.
Please go to Dashboard > Products > Categories > edit category > add Thumbnail image.
If you have any additional questions or concerns, please do not hesitate to reach out to us. Our support team is always available to assist you.
Kind Regards,
8theme team