jclouds restricts AMI owners to Amazon, Alestic, Canonical, and Rightscale to avoid slow image ID listings. This can be changed (to pick up your own AMIs) by setting jclouds.ec2.ami-owners to a comma-separated list of owner IDs.
ComputeServiceContextBuilder needs to construct a ComputeServiceContext which passes a properties file with this property from the Whirr configuration. In fact, it should probably pass all properties that start with "jclouds." to ComputeServiceContextBuilder.