Blocking the full size image

We have an omeka project with scanned historical images, some of which come from a historical society who specifically requested that the full size image not be available for public display.

Is there a simple way to flag certain items as not having the full size image be clickable? We don't need a complicated solution that requires two file versions or anything, just something as simple as blocking the full size URL image would do.

Is this possible?

In more technical terms, on the item page, we need the div id="item-images" to only display the thumbnail image without any sort of a-href linking to the full size original image.

And hopefully we can achieve this only on certain images which we have flagged a certain way.

Also, we would need this to not involve editing core in any way. It would have to be accomplished in the theme.

Is there any identifying metadata that goes along with these pictures? If so, you could probably write a conditional statement something like:

if collection = this historical society then display small image else display normal image

However anyone familiar with Omeka could get the image's file name and then switch the directory to the file directory which contains full size original images.