The migration tool doesn't guarantee the avatar migration. phpBB stores avatars in different ways, so it could be missed as well. In any case, please leave the wpForo forum URL we'll take a look at the missing avatar URLs If the avatar URL exists, but they don't load images then I assume you've not moved the avatars from phpBB directory to WordPress.
If the source URLs doesn't exist, then the tool could not find and migrate avatars. You can use Leira Letter Avatar plugin to fill all users avatars with the first name of their usernames. It's a nice replacement, once it's installed, go to the plugin settings and enable it.
In case you want to say thank you !)
We'd really appreciate and be thankful if you leave a good review on plugin page. This is the best way to say thank you to this project and support team.