This was an interesting find by one of my colleagues, and verified by myself to be a real issue.
All of a sudden, one of our custom entities within our development environment started to throw errors when trying to edit business rules.
We got to the point where we were seriously considering resetting the environment using a backup, when it was discovered that the only thing we could think that had changed was that the entity had been enabled for Entity Images as a test. Sure enough, deleting the field that was created for the entity image was enough to bring the business rules back.
So, let that be a lesson, if you enable entity images on a custom entity, you may just lose access to your business rules.
I am not sure if Microsoft is aware of the issue, or are planning on fixing it, but its something to watch out for.