I did my best to see what could be the issue – disabled all plugins and tried to import too but no matter what we do, no image gets imported for some reason. This happens even when I try to import by invoking the twenty fourteen default theme. Have not come across this situation before.
Fortunately, we are able to set Featured Image on the blog posts, portfolio entries etc. and they show up in the respective pages and posts without issues. In your case, you just need to set the featured image manually for all posts/portfolio items(which you would need to anyway since images we provide are just placeholders). Sorry, could not help more on this. You can try reinstalling WordPress once to check if that resolves the issue. Hope this helps.