ProviderImage and provider account association

[jprovaznik]

ProviderImage with provider account assocition is little bit confusing:
I can specify model which represents provider accounts by setting "Tim.provider_account_class"

but on the other side provider_account_id is of type string:
db/migrate/20120911204329_create_tim_provider_images.rb: t.string :provider_account_id

also populate_factory_fields method in ProviderImage model sets provider_account_id attribute directly from iamgefactory value.

expected behavior:
ProviderImage model should be ideally connected with a provider account model by common AR ids.