CRM: Pre and Post Entity Images

PreEntityImages and PostEntityImages contain snapshots of the primary entity's attributes before (pre) and after (post) the core platform operation. Microsoft Dynamics CRM populates the pre-entity and post-entity images based on the security privileges of the impersonated system user. Only entity attributes that are set to a value or null are available in the pre or post entity images. You can specify to have the platform populate these PreEntityImages and PostEntityImages properties when you register your plug-in. The entity alias value you specify during plug-in registration is used as the key into the image collection in your plug-in code.
There are some events where images are not available. For example, only synchronous post-event and asynchronous registered plug-ins have PostEntityImages populated. In addition, the create operation does not support a pre-image and a delete operation does not support a post-image.