I think the best practice is to use standard FileNet Image Services Resource Adapter (ISRA) if you can.
Product Documentation for FileNet Image Services Resource Adapter
Question
I'm currently trying to migrate one of my customer's ECM system from FileNet to Alfresco.
Since they are using Filenet Image Services 4.0 SP4, I cannot take advantage of CMIS (only available from 4.5.x).
According to this blog in more or less similar conditions :
The best practice seems to be reading the database to find all documents (data + link to image) :
Solution
I think the best practice is to use standard FileNet Image Services Resource Adapter (ISRA) if you can.
Product Documentation for FileNet Image Services Resource Adapter
OTHER TIPS
For content migrations to Alfresco you can use migration-center. You can download a free evaluation copy of migration-center on http://www.migration-center.com/free-evaluation-copy/