CRM 2013. Case create event (PreImage, PostImage etc)

Hi!

In continue of my question post.

Now i trying to create plugin calling event on case create (after it was created and saved). Plugin gets some data from some account entity and puts it into the case entity after it created. cases are created automatically.

And again faced with similar problem (the same error as in previous post).
Need a recommendation about correct settings of plugin event.

            Entity postImageEntity = (context.PostEntityImages != null && context.PostEntityImages.Contains(this.postImageAlias)) ? context.PostEntityImages[this.postImageAlias] : null;

            if (context.PostEntityImages.Contains("PostImage") && context.PostEntityImages["PostImage"] is Entity && context.Depth < 2)
            {
                try
                {
//do something
}}}