When you are importing demo content, sometimes you can see this – Failed to import ‘Your page name’ “name of the picture” like in the image below:
These errors can be easily ignored, as you’ll need to replace the grey placeholders that are imported. This sometimes happens because the server time outed while importing the placeholders images. But since they are placeholders, you’ll need to replace them anyhow. So you’re safe to ignore them, the rest of the content is imported normally.