Probably Great Before

This plugin seems like it probably did quite well earlier, but it has a couple downsides to it:

– It doesn’t bring in all images on each post, just the first one, so you may need to run it multiple times.
– It doesn’t add them to the media manager in WordPress, so you’ll need to use another extension like Media Sync to fix that after the fact.

If those aren’t dealbreakers, you can still get use out of the plugin even in WordPress 5. However I had better experience switching to the plugin “Cache Images” instead.