I made a test with my newly received Cardbox 3.0 I opened an V2 database, created a new image field, saved the native format. When trying the image features, draging an image (Jpeg) from my windows screen into the image field, it appears in my record. Saving with Ctrl S is fine. Then browsing to the next record and back again... the image is gone.
Cardbox3 can open and edit V2 databases, but you cannot edit a V2 database if it contains an image field. Images in Cardbox3 are stored in a different way. You'll have to convert your database to the Cardbox3 format using Tools > Management > Rebuild. It will only take a minute or so and you'll be fine.