It does look like something is either on the lens or somehow inside the actual sensor.

If so I would suggest in contacting the manufacturer (HTC) and ask about warranties about such a thing happening.

 

Truth to be told, I've got my second htc one under manufacturer warranty, due to the same problem as urs, and guess what, the new phone got its share of dirt inside the sensor, in less than a month... I'm not sure whether I should get it replaced again...