From time to time the following thing happens to people:
- They add an image field to an existing database.
- They try to add an image to the database.
- Cardbox reports that the image field is read-only.
Understandably, this is rather puzzling. It only happens with databases that were created with older versions of Cardbox. The quick cure is to rebuild the database with Tools > Management > Rebuild > Database. This Knowledge Base page gives a detailed explanation.