I would like to customize the behavior of "Image Not Found 404"

I have an image archive and some images link to a larger version while others do not, and just link to where that larger version will be...

I would like those that link to the fake url, to be spared the "Image Not Found 404"
instead I would like to be able to customize this error with something like "A full size version of this image is not yet available. Check back in the future or send us a request for a larger version of this image."
or alternatively, just have the images that have a 404 link do nothing when you click on it?

The do-nothing-on-click option would need to be implemented server side in php code or whatever server side language is in use. I would imagine that such an implementation would simply not write a link for an image that did not exist on the file system, but if it did, would add its own onclick handler in place of the 'floatbox' class name.

You can replace 404.jpg in the floatbox/graphics folder with whatever image you prefer and may want to place your custom error message into this replacement. It will of course be the same message for each unavailable image.