The site is perfectly usable. More than usable, even, because I get RES-style zooming for free, amongst other things.
You can right-click save normally already - just right-click and save. If you want the browser zoom tools, hit the "View in Browser" button or right-click and open in new tab
Nobody else has been able to reproduce it, and you haven't given enough information on what you're doing or what you've tried. Have you tried different machines? Different browsers? Have you tried pressing the various buttons in the UI? What kind of machine do you have? Do you have a very small amount of RAM? Et cetera.
You might say your browser works fine, but I'd put my money on something there being the issue.
I saw you tried different versions on 2 different machines -- it certainly suggests something related to your setup: I can zoom, open in new tab, save as, etc on Firefox 41 on Ubuntu 14.04.
Do you have running app for mouse gestures? That's the only thing I can think of that makes sense in your scenario. Try holding the right click for a few seconds, see if the context menu appears.
Which browser are you using that you can't use right click-> save on a blob URL'd image? It's using the standard HTML image and appears to be working in Chrome, Firefox and Edge.
A better solution would be decrypting the image and then loading the image into an image tag with a data: uri in base64 encoding.
I could zoom normally with the browsers own zoom tools, I could right-click save normally, etc.