Image occlusions are not a native feature of Anki as of today.
There are several add-ons for this, the most popular and feature-rich being Image Occlusion Enhanced for Anki 2.1 (alpha) - AnkiWeb.
Closet For Anki - AnkiWeb is a great option for people who want to reduce the amount of notetypes for their workflow, but it requires some effort to set up.