If you can't view the original size image at the true source of image, then it has been explicitly made that way by the server so that people who think they are smart can't get around it. It's quite common on sites that hold value in their images to produce two images on upload (Simple scripting job); The original to store privately, and a reduced quality preview copy (Perhaps even watermarked). The original needs no connection to the poor copy, so you're at a dead end if that's your case.
Of course, there are lots of lousy methods used to keep images safe too — generally obfuscation of some manner that can be by-passed — but I'm not all that interested in helping people jack other people's images.
@ LearningCoder: When you click "open image in new tab", it's probably respecting the HTML/CSS resizing or something.