Due to the lower resolution of the camera and the Duo camera requirements within the HTC default camera you cannot adjust the resolution for still pictures on this phone.
You can download the new Google camera and it allows you to adjust the camera with these options: 4.2, 2.1, 0.9 for the rear camera and 4.9, 1.9, and 1.2 for the front.
In order to access these options swipe to the right while in the camera app and press the settings icon, and press resolution and quality.
I hope this helps.