Don't know then. Makes absolutely no sense. Do you have a client mime override? Are you trying this from work? Try it from home or your phone?

I don't have that issue on any browser on any PC.

Unless you're right clicking on the image itself and choosing "View Image"...

Should just be left-clicking on it which opens a new tab with the URL to full-sized image.

What happens if you right click and choose "Open In New Tab"?