A big difference between WordPress and others CMSs, like Drupal, is the fact that WordPress does not allow users to upload/change their profile picture. The reason is because the profile picture of a user is not managed by WordPress itself. Instead, WordPress relies on Gravatar (global avatar) which is a service to retrieve and show the picture corresponding to a given user.
The benefit of gravatars is that the user can share the same profile picture among most of the blogs, forums and wherever he/she participates. The disadvantage, instead, is that the user needs to register in Gravatar and upload his/her picture for you to see it in your WordPress.
This is the reason why user migration from Drupal, Joomla and others CMSs will not migrate the profile picture of each user to WordPress. If the migrated user is already registered in Gravatar (based on the email information), his/her picture will immediately appear in WordPress. Otherwise, he/she will need to register and upload the picture there.
Of course, if migrating user pictures is a must for you, there are WordPress plug-ins that provide an alternative functionality. We can help you choose one of them and analyze any additional migration features you will need to use and initialize with.
Posted in: Content export / import User pictures not showing up in WordPress