Prohibit access to full resolution image when using ->size()

[cabrooney]

I am using ->size() to downsize images on the website. I love to have the original images stored in the backend for possible future changes in resolution.
Visitors can access the original (full resolution) images by deleting the size numbers in the url of the image.

Is it possible to prohibit access to the original images? 

[horst]

I do it like this: