Does the user need to modify this field at all? If not, use the behavoir settings in the Inspectors data tab to deny access to this field while in browse mode. You can use a separate layout for inserting new images and for deleting them when that's truly something you want to permit. (And manage security can be used to control which users are authorized to do so.)
Since you appear to be creating a database where you will insert large numbers of images into containers, make sure to investigate the "store by reference" option and the nuts and bolts in how it works when the file is shared over a network and when accessed by client machines of both windows and Mac platforms.