A big difference of WordPress wrt others CMSs like Drupal is the fact that WordPress does not allow users to upload/change their profile picture. The reason being that the profile picture of a user is not managed by WordPress itself. Instead, WordPress relies on the Gravatar (global avatar) service to retrieve and show the picture corresponding to a given user.
The benefit of gravatars is that you can share the same profile picture among most of the blogs, forums, … where you participate. The disadvantage is that you need to register in Gravatar and upload your picture to see it in your WordPress.
This is the reason why user migration from Drupal, Joomla and others to WordPress 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. If not, s/he 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 and study the additional migration features you’ll need to use/initialize it.
Posted in: Content export / import User pictures not showing up in WordPress