The alternative is that the browser does a purely raster zoom ... which gives you horizontal scrolling.
I'm fairly certain that the vast majority of people who zoom their browser don't expect horizontal scrolling to happen...
There's a few solutions if you want to zoom an image. All of which are at the user end.
1. Open the image in a new tab. Which makes sense for a website that allows user-submitted images, anyway. This is why the image upload form automatically linkifies the image to do that.
I do that all the time to view at original image resolution.
2. Install an extension that displays an image at original res, via overlay.
The only thing a site like this can do is display an image in a lightbox on click. Which I personally hate - it's always clunky to use. Some forums do this.
Much easier to just open in a separate tab.