By "static images" I meant they're already in the deployment as images and that they're not created through code. The URLs for static and dynamically created images don't differ that much. I created the image generator ActionBean so that its URL is like pointing to a "static" image.
As for the generator being called on conversion, I found out--thanks to Lester's suggestion--that it's NOT being called. The thing is: there's an interceptor in the application that limits access to ActionBeans to logged in users and the HTTPClient is not set to be logged in.
I could probably work with a solution from here. Thanks.
EDIT: it probably isn't the HTTPClient that's not set to be logged in exactly but some other internal component/s.