Why are my long images so narrow?

Whenever I click on an image that is too tall, it ends up incredibly narrow. This is typically on r/tumblr, where they post long post conversations, and they end up unreadably small. As far as I can tell, there's no way to zoom in. I've tried zooming in through my browser, but that doesn't change the size of the image.

An example

Any idea how to fix this?

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